target/99660 - missing VX_CPU_PREFIX for vxworksae

target/99660 - missing VX_CPU_PREFIX for vxworksae

This fixes an oversight which causes make all-gcc
to fail for --target=*vxworksae or vxworksmils, a regression
introduced by the recent VxWorks7 related updates.

Both AE and MILS variants resort to a common config/vxworksae.h,
which misses a definition of VX_CPU_PREFIX expected by port
specific headers.

The change just provides the missing definition.

2021-03-19  Olivier Hainque  <hainque@adacore.com>

gcc/
	PR target/99660
	* config/vxworksae.h (VX_CPU_PREFIX): Define.