[powerpc] libc_feupdateenv_test: optimize FPSCR access

ROUND_TO_ODD and a couple of other places use libc_feupdateenv_test to
restore the rounding mode and exception enables, preserve exception flags,
and test whether given exception(s) were generated.

If the exception flags haven't changed, then it is sufficient and a bit
more efficient to just restore the rounding mode and enables, rather than
writing the full Floating-Point Status and Control Register (FPSCR).

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