[compiler-rt][hwasan] Setup hwasan thread handling on Fuchsia

This patch splits up hwasan thread creation between `__sanitizer_before_thread_create_hook`,
`__sanitizer_thread_create_hook`, and `__sanitizer_thread_start_hook`.
The linux implementation creates the hwasan thread object inside the
new thread. On Fuchsia, we know the stack bounds before thread creation,
so we can initialize part of the thread object in `__sanitizer_before_thread_create_hook`,
then initialize the stack ring buffer in `__sanitizer_thread_start_hook`
once we enter the thread.

Differential Revision: https://reviews.llvm.org/D104085