Preserve cpu specific CRTSTUFF_T_CFLAGS on powerpc-vxworks7

Preserve cpu specific CRTSTUFF_T_CFLAGS on powerpc-vxworks7

The unconditional assignment performed in t-vxworks to handle
include flags currently overrides what specific cpu ports had
for the regular (!vxworks) crtstuff objects.

This was not done on purpose and the proposed change adjusts the
configuration bits to apply the vxworks specific flags on top of
the cpu ones instead.

2021-12-07  Olivier Hainque  <hainque@adacore.com>

	* config.host (powerpc*-wrs-vxworks7*): Place t-crtstuff
	ahead of the other files in tmake_files.
	* config/t-vxworks: Add to CRTSTUFF_T_CFLAGS instead of
	overriding it.