atom: Consolidate cl_khr_{local,global}_int32_{base,extended}_atomics implementation

These are just atomic_* wrappers.
Switch inc, dec to use atomic_* wrappers as well.

Reviewed-By: Aaron Watry <awatry@gmail.com>
Tested-By: Aaron Watry <awatry@gmail.com>
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
llvm-svn: 335279