libc: fix an unused-value error in LOCK/UNLOCK

According to http://gcc.gnu.org/gcc-4.9/porting_to.html

, gcc now warns
about unused right-hand side of a comma expression that contains no side
effects.
Rewrite LOCK/UNCLOCK using inline funciton instead of comma expression.

Signed-off-by: Boqun Feng <boqun.feng@linux.vnet.ibm.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>