testsuite: arm: Use effective-target for pure-code/* tests

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

gcc/testsuite/ChangeLog:

	* gcc.target/arm/pure-code/no-literal-pool-m0.c: Use
	effective-target arm_cpu_cortex-m0.
	* gcc.target/arm/pure-code/no-literal-pool-m23.c: Use
	effective-target arm_cpu_cortex-m23.
	* gcc.target/arm/pure-code/pr94538-1.c: Likewise.
	* gcc.target/arm/pure-code/pr109800.c: Use effective-target
	arm_arch_v7em_hard.
	* lib/target-supports.exp: Define effective-target
	arm_cpu_cortex_m0, arm_cpu_cortex_m23 and arm_arch_v7em_hard.

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