testsuite: arm: Use effective-target for {gcc,g++}.target/arm/ tests

testsuite: arm: Use effective-target for {gcc,g++}.target/arm/ tests

Update test cases to use -mcpu=unset/-march=unset feature introduced in
r15-3606-g7d6c6a0d15c.

gcc/testsuite/ChangeLog:

	* g++.target/arm/pr103676.C: Use effective-target
	arm_cpu_cortex_m7.
	* gcc.target/arm/no-volatile-in-it.c: Likewise.
	* gcc.target/arm/fma-sp.c: Use effective-target
	arm_cpu_cortex_m4_hard.
	* gcc.target/arm/pr53859.c: Use effective-target
	arm_cpu_cortex_m4.
	* gcc.target/arm/mve/intrinsics/pr97327.c: Use effective-target
	arm_cpu_cortex_m55.
	* gcc.target/arm/pr65067.c: Use effective-target
	arm_cpu_cortex_m3.
	* lib/target-supports.exp: Define effective-target
	arm_cpu_cortex_m3, arm_cpu_cortex_m4, arm_cpu_cortex_m4_hard,
	arm_cpu_cortex_m7 and arm_cpu_cortex_m55.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>