hwasan: Move stack ring buffer initialization before InitStackAndTls.

D104248 moved the call to GetThreadStackAndTls to before the
initialization of the ring buffer TLS slot. As a result, if libc
is instrumented we crash in pthread_getattr_np which is called from
__sanitizer::GetThreadStackTopAndBottom.

Fix the problem by moving the stack ring buffer initialization before
the call to InitStackAndTls.

Differential Revision: https://reviews.llvm.org/D108184