[libc] Add compile options to pthread_create target.

[sivachandra]

Summary

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.

[michaelrj]

LGTM

libc/src/pthread/CMakeLists.txt

add_entrypoint_object(
   SRCS
     pthread_create.cpp
   HDRS
     pthread_create.h
   DEPENDS
     libc.include.errno
     libc.include.pthread
     libc.src.__support.threads.thread
+  COMPILE_OPTIONS
+    -O3
+    -fno-omit-frame-pointer
 )
 
 add_entrypoint_object(
   pthread_join
   SRCS
     pthread_join.cpp
   HDRS
     pthread_join.h
   DEPENDS
     libc.include.pthread
     libc.src.__support.threads.thread
 )