[PowerPC] Expose some calling convention functions in PPCISelLowering.h.

This is a preparatory patch for fast-isel support.  The instruction
selector will need to access some functions in PPCGenCallingConv.inc,
which in turn requires several helper functions to be defined.  These
are currently defined near the only use of PCCGenCallingConv.inc,
inside PPCISelLowering.cpp.  This patch moves the declaration of the
functions into the associated header file to provide the needed
visibility.

No functional change intended.