powerpc: Fix build failures with current GCC

Since GCC commit 271500 (svn), also known as the following commit on the
git mirror:

commit 61edec870f9fdfb5df3fa4e40f28cbaede28a5b1
Author: amodra <amodra@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Wed May 22 04:34:26 2019 +0000

    [RS6000] Don't pass -many to the assembler

glibc builds are failing when an assembly implementation does not
declare the correct '.machine' directive, or when no such directive is
declared at all.  For example, when a POWER6 instruction is used, but
'.machine power6' is not declared, the assembler will fail with an error
similar to the following:

    ../sysdeps/powerpc/powerpc64/power8/strcmp.S: Assembler messages:
     24 ../sysdeps/powerpc/powerpc64/power8/strcmp.S:55: Error: unrecognized opcode: `cmpb'

This patch adds '.machine powerN' directives where none existed, as well
as it updates '.machine power7' directives on POWER8 files, because the
minimum binutils version required to build glibc (binutils 2.25) now
provides this machine version.  It also adds '-many' to the assembler
command used to build tst-set_ppr.c.

Tested for powerpc, powerpc64, and powerpc64le, as well as with
build-many-glibcs.py for powerpc targets.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>