[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