testsuite: Add arm_arch_v7a_ok effective-target to pr57351.c

testsuite: Add arm_arch_v7a_ok effective-target to pr57351.c

I've noticed that overriding cpu/arch flags when running the testsuite
can cause this test to fail rather than being skipped because of
incompatible flags combination.

Since the test forces -march=armv7-a, make sure it is accepted in
combination with the current runtestflags.

2021-07-08  Christophe Lyon  <christophe.lyon@foss.st.om>

	gcc/testsuite/
	* gcc.dg/debug/pr57351.c: Require arm_arch_v7a_ok
	effective-target.