powerpc: Cleanup: use actual power8 assembly mnemonics

Some implementations in sysdeps/powerpc/powerpc64/power8/*.S still had
pre power8 compatible binutils hardcoded macros and were not using
.machine power8.

This patch should not have semantic changes, in fact it should have the
same exact code generated.

Tested that generated stripped shared objects are identical when
using "strip --remove-section=.note.gnu.build-id".

Checked on:
- powerpc64le, power9, build-many-glibcs.py, gcc 6.4.1 20180104, binutils 2.26.2.20160726
- powerpc64le, power8, debian 9, gcc 6.3.0 20170516, binutils 2.28
- powerpc64le, power9, ubuntu 19.04, gcc 8.3.0, binutils 2.32
- powerpc64le, power9, opensuse tumbleweed, gcc 9.1.1 20190527, binutils 2.32
- powerpc64, power9, debian 10, gcc 8.3.0, binutils 2.31.1

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Reviewed-by: Gabriel F. T. Gomes <gabrielftg@linux.ibm.com>