Modified MutexLock to use a pthread mutex on Android.

Modified MutexLock to use a pthread mutex on Android.

The BackoffLock spins idle for a while when waiting for a locked mutex
before yielding the core, thus wasting many CPU cycles. Modern pthread
implementations have low overhead mutexes which make the thread sleep
if the lock is already held, and efficiently resume them it becomes
available.

Change-Id: I26b64c86db620739671373fd0d82085744d34fa8
Reviewed-on: https://swiftshader-review.googlesource.com/8648
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>