Fix asm constraint of compare_and_swap for clang-3.1/mips

* src/atomic_ops/sysdeps/gcc/mips.h (AO_compare_and_swap): Use "+m"
asm constraint for *addr instead of "+R" (the letter is unsupported
by clang3.1 resulting in "invalid output constraint in asm" error).
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).
Fix asm constraint of CAS memory operand for gcc/alpha

* src/atomic_ops/sysdeps/gcc/alpha.h (AO_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).