testsuite: arm: Use effective-target for thumb2-slow-flash-data* tests

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

gcc/testsuite/ChangeLog:

	* gcc.target/arm/thumb2-slow-flash-data-2.c: Use
	effective-target arm_arch_v7em_hard.
	* gcc.target/arm/thumb2-slow-flash-data-3.c: Likewise.
	* gcc.target/arm/thumb2-slow-flash-data-4.c: Likewise.
	* gcc.target/arm/thumb2-slow-flash-data-5.c: Likewise.

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