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

This has been used in gcc-9 based production compilers for a
year on both vxworks 7 and 6.9. Also passed a build & test sequence
for powerpc-vxworks 7 and 6.9 with gcc-10 and a sanity check build
with a recent mainline.

Will commit to mainline shortly.

Olivier

2020-10-01  Olivier Hainque  <hainque@adacore.com>

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