MFC r341751-r341752, r342025, r342107

r341751:
powerpc/SPE: Reload vector registers after efdabs/efdnabs/efdneg

r341752:
powerpc/SPE: Copy lower part of source register to target for
efdabs/efdnabs/efdneg

r342025:
powerpcspe: Fix GPR handling in SPE exception handler

r342107:
powerpcspe: Don't require FPU_EMU for powerpcspe IEEE emulation