MIPS: define local_xchg from xchg_local to atomic_long_xchg
Perf-events is now using local_t helper functions internally. There is a
use of local_xchg(). On MIPS, this is defined to xchg_local() which is
missing in asm/system.h. This patch re-defines local_xchg() in asm/local.h
to atomic_long_xchg(). Then Perf-events can pass the build.