MIPS: define local_xchg from xchg_local to atomic_long_xchg

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.

Signed-off-by: Deng-Cheng Zhu <dengcheng.zhu@gmail.com>
To: linux-mips@linux-mips.org
Cc: a.p.zijlstra@chello.nl
Cc: paulus@samba.org
Cc: mingo@elte.hu
Cc: acme@redhat.com
Cc: jamie.iles@picochip.com
Cc: ddaney@caviumnetworks.com
Cc: matt@console-pimps.org
Patchwork: https://patchwork.linux-mips.org/patch/1687/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>