Leverage VX_CPU_PREFIX in aarch64-vxworks.h

Leverage VX_CPU_PREFIX in aarch64-vxworks.h

This change tightens the CPU macro definitions issued
for VxWorks system headers on aarch64 to incorporate
the common VX_CPU_PREFIX facility, as the powerpc port
does.

The net effect for current configurations is the addition
of an actual "_VX_" prefix to the references to architecture
representative values. The absence of this prefix is most
often compensated for in system headers, but not always (when
going through particular #include paths), and this caused
a couple of spurious test failures.

2021-12-09  Olivier Hainque  <hainque@adacore.com>

gcc/
	* config/aarch64/aarch64-vxworks.h (TARGET_OS_CPP_BUILTINS):
	Use VX_CPU_PREFIX in CPU definitions.