Fix asm constraint of fetch_and_add, test_and_set, fetch_CAS for MIPS (gcc)

Fix asm constraint of fetch_and_add, test_and_set, fetch_CAS for MIPS (gcc)

* src/atomic_ops/sysdeps/gcc/mips.h (AO_fetch_and_add,
AO_test_and_set, AO_fetch_compare_and_swap): Use "+m" asm constraint
for *addr instead of "=m" (because the value pointed by addr is read
and written by the code).