[powerpc] get_rounding_mode: utilize faster method to get rounding mode

Add support to use 'mffsl' instruction if compiled for POWER9 (or later).

Also, mask the result to avoid bleeding unrelated bits into the result of
_FPU_GET_RC().

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>