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.