Add atomic helper functions for arm.

1. Implement some gcc __sync_* builtins for compatibility

See http://gcc.gnu.org/onlinedocs/gcc-4.8.0/gcc/_005f_005fsync-Builtins.html
Note that only 4 bytes operation is supported now.
The reference implementation: AOSP/system/core/include/cutils/atomic-arm.h

2. Alias __aeabi_fcmpun to __unordsf2.

Change-Id: Id44293824b9011876e22dfdefc7b241dfb47e89d
2 files changed