Migration from llvm_asm! to asm!

[tesuji]

Not really a high priority task, but it would be nice to see official rust crates
using new recommended language features over old ones.

Related issues/PRs:

• Use llvm_asm! instead of asm! #846
• Tracking Issue for inline assembly (asm!) rust#72016

[bjorn3]

This would be nice for cg_clif as only asm! is implemented.

[tesuji]

As noted in rust-lang/rust#76669 (comment) , we have to support LLVM 8 (which is currently the minimum supported version).
We have to use ATT syntax for x86.

[tesuji]

Note: Some architectures currently aren't supported by asm!: mips, ppc, ...
So if the llvm_asm code don't have target_arch or target_arch only on those archs,
we have to continue using llvm_asm.

[josephlr]

It looks like all of the current llvm_asm! uses in core_arch and std_detect are on arm, aarch64, x86, or x86_64, so we should be able to switch all of them to asm!, using att-syntax on 32-bit x86.

The only other use is in assert-instr-macro, where it's not used for assembly, but as a "volatile" fence. This is similar to how libcore implements black_box. So perhaps we can just use that instead.