[Ada] Map gnatlib-shared to gnatlib-shared-dual for aarch64-vx7r2

[Ada] Map gnatlib-shared to gnatlib-shared-dual for aarch64-vx7r2

This is an incremental change towards supporting shared libraries
for VxWorks on aarch64.

The aarch64-vx7r2 compiler supports compilation with -fpic/PIC.  This
change adds aarch64 to the list of CPUs for which GNATLIB_SHARED maps to
gnatlib-shared-dual for vxworks7r2, so "make gnatlib-shared" actually
builds a shared lib.

While other adjustments will be needed to get the runtime tests to pass,
this one is a necessary step and doesn't impair the rest.

gcc/ada/

	* Makefile.rtl: Add aarch64 to the list of CPUs for which
	GNATLIB_SHARED maps to gnatlib-shared-dual for vxworks7r2.