make mips64 a_sc_p atomic primitive's asm constraints work with clang
make mips64 a_sc_p atomic primitive's asm constraints work with clang

apparently clang does not accept matching-register input and output
constraints that differ in size (32-bit vs 64-bit).

based on patch by Jaydeep Patil.

chardi/musl,chardi/musl,chardi/musl