powerpcspe: Fix GPR handling in SPE exception handler

Optimize the exception handler to only save and load the upper word of the
GPRs used in the emulating instruction. This reduces the save/load
overhead, and as a side effect does not overwrite the upper word of any
temporary register.

With this commit I am now able to run editors/abiword and math/gnumeric on a
e500-based system.

MFC after: 1 week
MFC With: r341752,r341751