MIPS: Fix MSA toolchain support detection.

The '-mmsa' option is not supported by all current compilers. Change
the detection to instead pass the option directly to the assembler.

Signed-off-by: Steven J. Hill <Steven.Hill@imgtec.com>
Reviewed-by: Paul Burton <Paul.Burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/6835/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>