[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

libc/src/pthread/CMakeLists.txt

@@ -253,6 +253,9 @@ add_entrypoint_object(
     libc.include.errno
     libc.include.pthread
     libc.src.__support.threads.thread
+  COMPILE_OPTIONS
+    -O3
+    -fno-omit-frame-pointer
 )
 
 add_entrypoint_object(