[powerpc] __fesetround_inline optimizations

On POWER9, use more efficient means to update the 2-bit rounding mode
via the 'mffscrn' instruction (instead of two 'mtfsb0/1' instructions
or one 'mtfsfi' instruction that modifies 4 bits).

Suggested-by: Paul E. Murphy <murphyp@linux.ibm.com>
Reviewed-By: Paul E Murphy <murphyp@linux.ibm.com>