powerpc: Fix unrecognized instruction errors with recent GCC

Recent binutils commit b25f942e18d6ecd7ec3e2d2e9930eb4f996c258a
changes the behavior of `.machine` directives to override, rather
than augment, the base CPU. This can result in _reduced_ functionality
when, for example, compiling for default machine "power8", but explicitly
asking for ".machine power5", which loses Altivec instructions.

In tst-ucontext-ppc64-vscr.c, while the instructions provoking the new
error messages are bracketed by ".machine power5", which is ostensibly
Power ISA 2.03 (POWER5), the POWER5 processor did not support the
VSX subset, so these instructions are not recognized as "power5".

Error: unrecognized opcode: `vspltisb'
Error: unrecognized opcode: `vpkuwus'
Error: unrecognized opcode: `mfvscr'
Error: unrecognized opcode: `stvx'

Manually adding the VSX subset via ".machine altivec" is sufficient.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>
(cherry picked from commit 064b475a)