arm: testsuite: tidy up pre-run check for g2.c

arm: testsuite: tidy up pre-run check for g2.c

gcc.target/arm/g2.c is an xscale-only test, but the test is quite old
and we have improved the infrastructure for setting up such tests now.
So make use of that to reduce the number of cases where this test fails
to run.

gcc/testsuite:

	* lib/target-supports.exp (check_effective_target_arm_arch_FUNC_ok):
	Add entry to check for xscale.
	* gcc.target/arm/g2.c: Use it.