Provide prebuilt std artifacts for sanitizers

[Gankra]

Based off of discussion in zulip

Rust currently has unstable support for sanitizers (#39699), but some sanitizers require (or prefer) std to be built with special instrumentation. The unstable cargo -Zbuild-std feature was introduced to address this issue, allowing you to ask cargo to rebuild std with your current RUSTFLAGS.

However, this is a bit messy to work with, and produces a lot of redundant work. Especially in the context of CI with vendored environments. It would be nice if prebuilt stds were provided for some configurations.

Sanitizers that merit consideration:

• ThreadSanitizer (tsan) -- requires an instrumented build to work right
• MemorySanitizer (msan) -- requires an instrumented built to work right
• AddressSanitizer (asan) -- can produce more accurate results with an instrumented built

For reference, Firefox is currently only using the -Zsanitizer flag for its "linux64" tsan builds, which I believe are currently debian8-amd64 machines. We have asan tasks in many different configurations, but evidently we haven't yet bothered to use -Zsanitizer for them (presumably because it's not required).

It has been suggested to provide builds for all the tier1 platforms, but I don't personally have an opinion on that.

[Gankra]

Note: some hackery may still be required to avoid proc macros/build.rs from using the tsan std, as rustc will still be uninstrumented and link into them (and there will be missing tsan symbols).

[briansmith]

Another possibility is to just do -Zbuild-std implicitly when a (stable) sanitizer is requested. This would require sanitizers to depend on the rust-src component, but I don't think that would be too terrible.