MIPS assembly pack: adapt it for MIPS[32|64]R6.

MIPS[32|64]R6 is binary and source incompatible with previous MIPS ISA
specifications. Fortunately it's still possible to resolve differences
in source code with standard pre-processor and switching to trap-free
version of addition and subtraction instructions.

Reviewed-by: Richard Levitte <levitte@openssl.org>