Rework CPP_BUILTINS_SPEC for powerpc-vxworks

This change reworks CPP_BUILTINS_SPEC for powerpc-vxworks to
prepare for the upcoming addition of 32 and 64 bit ports for
VxWorks 7r2.

2020-10-14  Olivier Hainque  <hainque@adacore.com>

gcc/
	* config/rs6000/vxworks.h (TARGET_OS_CPP_BUILTINS): Accommodate
	expectations from different versions of VxWorks, for 32 or 64bit
	configurations.