[arm] require armv7 support for [PR120424]

[arm] require armv7 support for [PR120424]

Without stating the architecture version required by the test, test
runs with options that are incompatible with the required
architecture version fail, e.g. -mfloat-abi=hard.

armv7 was not covered by the long list of arm variants in
target-supports.exp, so add it, and use it for the effective target
requirement and for the option.

for  gcc/testsuite/ChangeLog

	PR rtl-optimization/120424
	* lib/target-supports.exp (arm arches): Add arm_arch_v7.
	* g++.target/arm/pr120424.C: Require armv7 support.  Use
	dg-add-options arm_arch_v7 instead of explicit -march=armv7.