arm: testsuite: improve compatibility of gcc.dg/debug/pr57351.c

This test is arm specific and requires neon.  To improve compatibility
add a new test for armv7-a with neon and use that.

gcc/testsuite:

	* lib/target-supports.exp (v7a_neon): New feature-test target.
	* gcc.dg/debug/pr57351.c: Use it.