Make libunwind build hermetic

[petrhosek]

We want to avoid exporting any symbols from Rust's version of libunwind,
and to do so we need to disable visibility annotations to make sure that
the -fvisibility=hidden has effect, and also hide global new/delete.

This matches the CMake build of libunwind.

[rust-highfive]

r? @nikomatsakis

(rust_highfive has picked a reviewer for you, use r? to override)

[petrhosek]

r? @tmandry

[rust-highfive]

The job mingw-check of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

##[section]Starting: Linux mingw-check
##[section]Starting: Initialize job
Agent name: 'Azure Pipelines 67'
Agent machine name: 'fv-az619'
Current agent version: '2.169.1'
##[group]Operating System
16.04.6
LTS
LTS
##[endgroup]
##[group]Virtual Environment
Environment: ubuntu-16.04
Version: 20200517.1
Included Software: https://github.com/actions/virtual-environments/blob/ubuntu16/20200517.1/images/linux/Ubuntu1604-README.md
##[endgroup]
Agent running as: 'vsts'
Prepare build directory.
Set build variables.
Download all required tasks.
Download all required tasks.
Downloading task: Bash (3.163.2)
Checking job knob settings.
   Knob: AgentToolsDirectory = /opt/hostedtoolcache Source: ${AGENT_TOOLSDIRECTORY} 
   Knob: AgentPerflog = /home/vsts/perflog Source: ${VSTS_AGENT_PERFLOG} 
Start tracking orphan processes.
##[section]Finishing: Initialize job
##[section]Starting: Configure Job Name
==============================================================================
---
========================== Starting Command Output ===========================
[command]/bin/bash --noprofile --norc /home/vsts/work/_temp/4394de8a-9282-46f3-aad5-d25ebf058c0b.sh

##[section]Finishing: Disable git automatic line ending conversion
##[section]Starting: Checkout rust-lang/rust@refs/pull/72746/merge to s
Task         : Get sources
Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
Version      : 1.0.0
Author       : Microsoft
---
##[command]git remote add origin https://github.com/rust-lang/rust
##[command]git config gc.auto 0
##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
##[command]git config --get-all http.proxy
##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/72746/merge:refs/remotes/pull/72746/merge
---
 ---> 3adb0605cc65
Step 6/7 : ENV RUN_CHECK_WITH_PARALLEL_QUERIES 1
 ---> Using cache
 ---> 28dbc326cb7f
Step 7/7 : ENV SCRIPT python3 ../x.py test src/tools/expand-yaml-anchors &&            python3 ../x.py check --target=i686-pc-windows-gnu --host=i686-pc-windows-gnu &&            python3 ../x.py build --stage 0 src/tools/build-manifest &&            python3 ../x.py test --stage 0 src/tools/compiletest &&            python3 ../x.py test src/tools/tidy &&            python3 ../x.py doc --stage 0 src/libstd &&            /scripts/validate-toolstate.sh
 ---> 537a01811900
Successfully built 537a01811900
Successfully tagged rust-ci:latest
Built container sha256:537a018119009dc218456238dec90b5530050db1e2a1e166550c218003f6159d
---
   Compiling autocfg v0.1.7
   Compiling std v0.0.0 (/checkout/src/libstd)
   Compiling compiler_builtins v0.1.28
   Compiling unwind v0.0.0 (/checkout/src/libunwind)
error: expected one of `.`, `;`, `?`, `}`, or an operator, found `cfg`
   |
   |
93 |             cfg.flag_if_supported("-fvisibility-global-new-delete-hidden")
   |                                                                           - expected one of `.`, `;`, `?`, `}`, or an operator
94 |             cfg.define("_LIBUNWIND_DISABLE_VISIBILITY_ANNOTATIONS", None);

error: aborting due to previous error

error: could not compile `unwind`.
---
  local time: Fri May 29 18:23:53 UTC 2020
  network time: Fri, 29 May 2020 18:23:53 GMT
== end clock drift check ==

##[error]Bash exited with code '1'.
##[section]Finishing: Run build
##[section]Starting: Checkout rust-lang/rust@refs/pull/72746/merge to s
Task         : Get sources
Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
Version      : 1.0.0
Author       : Microsoft
Author       : Microsoft
Help         : [More Information](https://go.microsoft.com/fwlink/?LinkId=798199)
==============================================================================
Cleaning any cached credential from repository: rust-lang/rust (GitHub)
##[section]Finishing: Checkout rust-lang/rust@refs/pull/72746/merge to s
Cleaning up task key
Start cleaning up orphan processes.
Terminate orphan process: pid (8327) (python)
##[section]Finishing: Finalize Job

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @rust-lang/infra. (Feature Requests)

[tmandry]

@bors r+ rollup=never

cc @Mark-Simulacrum

[bors]

📌 Commit 21abc88 has been approved by tmandry

[Dylan-DPC-zz]

@bors p=1

[jethrogb]

This matches the CMake build of libunwind.

As asked previously, why don't you just use CMake to do the build? That avoids having to maintain a separate build flow.

[bors]

⌛ Testing commit 21abc88 with merge 6f48888...

[bors]

☀️ Test successful - checks-azure
Approved by: tmandry
Pushing 6f48888 to master...