powerpc: Fix unrecognized instruction errors with recent binutils

Recent versions of binutils (with commit
b25f942e18d6ecd7ec3e2d2e9930eb4f996c258a) stopped preserving "sticky"
options across a base `.machine` directive, nullifying the use of
passing "-many" through GCC to the assembler.  As a result, some
instructions which were recognized even under older, more stringent
`.machine` directives become unrecognized instructions in that
context.

In `sysdeps/powerpc/tst-set_ppr.c`, the use of the `mfppr32` extended
mnemonic became unrecognized, as the default compilation with GCC for
32bit powerpc adds a `.machine ppc` in the resulting assembly, so the
command line option `-Wa,-many` is essentially ignored, and the ISA 2.06
instructions and mnemonics, like `mfppr32`, are unrecognized.

The compilation of `sysdeps/powerpc/tst-set_ppr.c` fails with:
Error: unrecognized opcode: `mfppr32'

Add appropriate `.machine` directives in the assembly to bracket the
`mfppr32` instruction.

Part of a 2019 fix (commit 9250e661) to
the above test's Makefile to add `-many` to the compilation when GCC
itself stopped passing `-many` to the assember no longer has any effect,
so remove that.

Reported-by: Joseph Myers <joseph@codesourcery.com>
(cherry picked from commit ee874f44)