[GWP-ASan] Fuchsia specific mapping & utilities functions

This CL introduces the Fuchsia versions of the existing platform
specific functions.

For Fuchsia, we need to track the VMAR (https://fuchsia.dev/fuchsia-src/reference/kernel_objects/vm_address_region)
of the Guarded Pool mapping, and for this purpose I added some platform
specific data structure that remains empty on POSIX platforms.

`getThreadID` is not super useful for Fuchsia so it's just left as a
stub for now.

While testing the changes in my Fuchsia tree, I realized that
`guarded_pool_allocator_tls.h` should have closed the namespace before
including `GWP_ASAN_PLATFORM_TLS_HEADER`, otherwise drama ensues.

This was tested in g3, upstream LLVM, and Fuchsia (with local changes).

Differential Revision: https://reviews.llvm.org/D90483