[POWERPC] Fix handling of stfiwx math emulation

Its legal for the stfiwx instruction to have RA = 0 as part of its
effective address calculation.  This is illegal for all other XE
form instructions.

Add code to compute the proper effective address for stfiwx if
RA = 0 rather than treating it as illegal.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>