libc: remove global lock from recursive mutex implementation.

This optimization improves the performance of recursive locks
drastically. When running the thread_stress program on a Xoom,
the total time to perform all operations goes from 1500 ms to
500 ms on average after this change is pushed to the device.

Change-Id: I5d9407a9191bdefdaccff7e7edefc096ebba9a9d
1 file changed