linux-user/ppc: Fix padding in mcontext_t for ppc64

linux-user/ppc: Fix padding in mcontext_t for ppc64

The padding that was added in 95cda4c44ee was added to a union,
and so it had no effect.  This fixes misalignment errors detected
by clang sanitizers for ppc64 and ppc64le.

In addition, only ppc64 allocates space for VSX registers, so do
not save them for ppc32.  The kernel only has references to
CONFIG_SPE in signal_32.c, so do not attempt to save them for ppc64.

Fixes: 95cda4c44ee
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200407032105.26711-1-richard.henderson@linaro.org>
Acked-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>