powerpcspe: Correct SPE high-component loading

Don't clobber the low part of the register restoring the high component of.
This could lead to very bad behavior if it's an ABI-affected register.

While here, also mark the asm volatile in the SPE high save case, to match
the load case.

Reported by: Branden Bergren (git_bdragon.rtk0.net)
MFC after: 1 week