[GWP-ASan] Abstract the thread local variables access

In a similar fashion to D87420 for Scudo, this CL introduces a way to
get thread local variables via a platform-specific reserved TLS slot,
since Fuchsia doesn't support ELF TLS from the libc itself.

If needing to use this, a platform will have to define
`GWP_ASAN_HAS_PLATFORM_TLS_SLOT` and provide `gwp_asan_platform_tls_slot.h`
which will define a `uint64_t *getPlatformGwpAsanTlsSlot()` function
that will return the TLS word of storage.

I snuck in a couple of cleanup items as well, moving some static
functions to anonymous namespace for consistency.

Differential Revision: https://reviews.llvm.org/D90195