arm: testsuite: improve compatibility of ftest-armv7m-thumb.c

This test is specific to armv7m cores which do not support hardware
floating-point.  We can improve its compatibility by having the default
options for this core specify -mfloat-abi=soft.

gcc/testsuite:

	* lib/target-supports.exp (check_effective_target_arm_arch_FUNC_ok):
	Use soft-float ABI for armv7m.
	* gcc.target/arm/ftest-armv7m-thumb.c: Use dg-require-effective-target
	to check flag compatibility.