testsuite: arm: Use effective-target for small-multiply-m* tests

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

gcc/testsuite/ChangeLog:

	* gcc.target/arm/small-multiply-m0-1.c: Use effective-target
	arm_arch_v6m and added option "-march=unset".
	* gcc.target/arm/small-multiply-m0-2.c: Likewise.
	* gcc.target/arm/small-multiply-m0-3.c: Likewise.
	* gcc.target/arm/small-multiply-m0plus-1.c: Likewise.
	* gcc.target/arm/small-multiply-m0plus-2.c: Likewise.
	* gcc.target/arm/small-multiply-m0plus-3.c: Likewise.
	* gcc.target/arm/small-multiply-m1-1.c: Likewise.
	* gcc.target/arm/small-multiply-m1-2.c: Likewise.
	* gcc.target/arm/small-multiply-m1-3.c: Likewise.
	* lib/target-supports.exp: Define effective-target
	arm_cpu_cortex_m0_small, arm_cpu_cortex_m0plus_small and
	arm_cpu_cortex_m1_small.

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