Fix potential race condition on CTS TC pthread_gettid_np

Root cause:
If start_routine thread exits before pthread_gettid_np is invokded, the "tid" field
will be cleared so that pthread_gettid_np will get "0" (which is cleared by kernel, 
due to the flag "CLONE_CHILD_CLEARTID" is set while calling clone system call inside
pthread_create).

Proposed patch:
Use a mutex to guarantee pthread_gettid_np will be invoked and returned before the
start_routine exits

Signed-off-by: Junjie Hu <junjie.hu@mediatek.com>

Change-Id: I22411f1b0f7446d76a0373cef4ccec858fac7018