target-ppc: Bug Fix: srawi

For 64 bit implementations, the special case of a shift by zero
should result in the sign extension of the least significant 32 bits
of the source GPR (not a direct copy of the 64 bit source GPR).

Example:

R3 A6212433228F41DC
srawi 3,3,0
R3 expected : 00000000228F41DC
R3 actual   : A6212433228F41DC (without this patch)

Signed-off-by: Tom Musta <tommusta@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>