[libc] Add compile options to pthread_create target.

The compile options now match that of thrd_create. Two compile options
are of importance:
1. -O3 - This is required so that stack is not used between the clone
   syscall and the start function in the child thread.
2. -fno-omit-frame-pointer - This is required so that we can sniff out
   the thread start args from the child thread's stack memory.

Without these two options, pthread_create will exhibit flaky behavior.

Reviewed By: lntue, michaelrj

Differential Revision: https://reviews.llvm.org/D127381