[compiler-rt] implement sigaltstack interception

Summary:
An implementation for `sigaltstack` to make its side effect be visible to MSAN.

```
ninja check-msan
```

Reviewers: vitalybuka, eugenis

Reviewed By: eugenis

Subscribers: dberris, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D73816

Patch by Igor Sugak.