libc: Fix recursive mutex lock implementation.

This fixes a bug that was introduced in the latest pthread optimization.
It happens when a recursive lock is contented by several threads. The main
issue was that the atomic counter increment in _recursive_increment() could
be annihilated by a non-conditional write in pthread_mutex_lock() used to
update the value's lower bits to indicate contention.

This patch re-introduces the use of the global recursive lock in
_recursive_increment(). This will hit performance, but a future patch
will be provided to remove it from the source code.

Change-Id: Ie22069d376cebf2e7d613ba00b6871567f333544
1 file changed