powerpc: Fix __fesetround_inline_nocheck on POWER9+ (BZ 31682)

The e68b1151 commit changed the
__fesetround_inline_nocheck implementation to use mffscrni
(through __fe_mffscrn) instead of mtfsfi.  For generic powerpc
ceil/floor/trunc, the function is supposed to disable the
floating-point inexact exception enable bit, however mffscrni
does not change any exception enable bits.

This patch fixes by reverting the optimization for the
__fesetround_inline_nocheck.

Checked on powerpc-linux-gnu.
Reviewed-by: Paul E. Murphy <murphyp@linux.ibm.com>