[GWP-ASan] Mutex implementation [2].

[GWP-ASan] Mutex implementation [2].

Summary:
See D60593 for further information.
This patch pulls out the mutex implementation and the required definitions file.

We implement our own mutex for GWP-ASan currently, because:

1. We must be compatible with the sum of the most restrictive elements of the supporting allocator's build system. Current targets for GWP-ASan include Scudo (on Linux and Fuchsia), and bionic (on Android).
2. Scudo specifies `-nostdlib++ -nonodefaultlibs`, meaning we can't use `std::mutex` or `mtx_t`.
3. We can't use `sanitizer_common`'s mutex, as the supporting allocators cannot afford the extra maintenance (Android, Fuchsia) and code size (Fuchsia) overheads that this would incur.

In future, we would like to implement a shared base mutex for GWP-ASan, Scudo and sanitizer_common. This will likely happen when both GWP-ASan and Scudo standalone are not in the development phase, at which point they will have stable requirements.

Reviewers: vlad.tsyrklevich, morehouse, jfb

Reviewed By: morehouse

Subscribers: dexonsmith, srhines, cfe-commits, kubamracek, mgorny, cryptoad, jfb, #sanitizers, llvm-commits, vitalybuka, eugenis

Tags: #sanitizers, #llvm, #clang

Differential Revision: https://reviews.llvm.org/D61923

llvm-svn: 362138