PPC32: Fix stack collision between FP and CR save areas.

The changes to CR spill handling missed a case for 32-bit PowerPC.
The code in PPCFrameLowering::processFunctionBeforeFrameFinalized()
checks whether CR spill has occurred using a flag in the function
info.  This flag is only set by storeRegToStackSlot and
loadRegFromStackSlot.  spillCalleeSavedRegisters does not call
storeRegToStackSlot, but instead produces MI directly.  Thus we don't
see the CR is spilled when assigning frame offsets, and the CR spill
ends up colliding with some other location (generally the FP slot).

This patch sets the flag in spillCalleeSavedRegisters for PPC32 so
that the CR spill is properly detected and gets its own slot in the
stack frame.