Fix unrecognised -mcpu target 'armv7-a' on arm-wrs-vxworks7

Fix unrecognised -mcpu target 'armv7-a' on arm-wrs-vxworks7

In the removal of arm-wrs-vxworks, the default cpu was updated from arm8
to armv7-a, but this is not recognized as a valid -mcpu target.  There
is however generic-armv7-a, which was likely the intended cpu that
should have been used instead.

gcc/ChangeLog:

	PR target/95420
	* config.gcc (arm-wrs-vxworks7*): Set default cpu to generic-armv7-a.