[powerpc] SET_RESTORE_ROUND improvements

[powerpc] SET_RESTORE_ROUND improvements

SET_RESTORE_ROUND uses libc_feholdsetround_ppc_ctx and
libc_feresetround_ppc_ctx to bracket a block of code where the floating point
rounding mode must be set to a certain value.

For the *prologue*, libc_feholdsetround_ppc_ctx is used and performs:
1. Read/save FPSCR.
2. Create new value for FPSCR with new rounding mode and enables cleared.
3. If new value is different than current value,
   a. If transitioning from a state where some exceptions enabled,
      enter "ignore exceptions / non-stop" mode.
   b. Write new value to FPSCR.
   c. Put a mark on the wall indicating the FPSCR was changed.

(1) uses the 'mffs' instruction.  On POWER9, the lighter weight 'mffsl'
instruction can be used, but it doesn't return all of the bits in the FPSCR.
fegetenv_status uses 'mffsl' on POWER9, 'mffs' otherwise, and can thus be
used instead of fegetenv_register.
(3b) uses 'mtfsf 0b11111111' to write the entire FPSCR, so it must
instead use 'mtfsf 0b00000011' to write just the enables and the mode,
because some of the rest of the bits are not valid if 'mffsl' was used.
fesetenv_mode uses 'mtfsf 0b00000011' on POWER9, 'mtfsf 0b11111111'
otherwise.

For the *epilogue*, libc_feresetround_ppc_ctx checks the mark on the wall, then
calls libc_feresetround_ppc, which just calls __libc_femergeenv_ppc with
parameters such that it performs:
1. Retreive saved value of FPSCR, saved in prologue above.
2. Read FPSCR.
3. Create new value of FPSCR where:
   - Summary bits and exception indicators = current OR saved.
   - Rounding mode and enables = saved.
   - Status bits = current.
4. If transitioning from some exceptions enabled to none,
   enter "ignore exceptions / non-stop" mode.
5. If transitioning from no exceptions enabled to some,
   enter "catch exceptions" mode.
6. Write new value to FPSCR.

The summary bits are hardwired to the exception indicators, so there is no
need to restore any saved summary bits.
The exception indicator bits, which are sticky and remain set unless
explicitly cleared, would only need to be restored if the code block
might explicitly clear any of them.  This is certainly not expected.

So, the only bits that need to be restored are the enables and the mode.
If it is the case that only those bits are to be restored, there is no need to
read the FPSCR.  Steps (2) and (3) are unnecessary, and step (6) only needs to
write the bits being restored.

We know we are transitioning out of "ignore exceptions" mode, so step (4) is
unnecessary, and in step (6), we only need to check the state we are
entering.