testsuite: arm: use effective-target for vsel*, mod* and pr65647.c tests

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

gcc/testsuite/ChangeLog

	* gcc.target/arm/pr65647.c: Use effective-target arm_arch_v6m.
	Removed unneeded dg-skip-if.
	* gcc.target/arm/mod_2.c: Use effective-target arm_cpu_cortex_a57.
	* gcc.target/arm/mod_256.c: Likewise.
	* gcc.target/arm/vseleqdf.c: Likewise.
	* gcc.target/arm/vseleqsf.c: Likewise.
	* gcc.target/arm/vselgedf.c: Likewise.
	* gcc.target/arm/vselgesf.c: Likewise.
	* gcc.target/arm/vselgtdf.c: Likewise.
	* gcc.target/arm/vselgtsf.c: Likewise.
	* gcc.target/arm/vselledf.c: Likewise.
	* gcc.target/arm/vsellesf.c: Likewise.
	* gcc.target/arm/vselltdf.c: Likewise.
	* gcc.target/arm/vselltsf.c: Likewise.
	* gcc.target/arm/vselnedf.c: Likewise.
	* gcc.target/arm/vselnesf.c: Likewise.
	* gcc.target/arm/vselvcdf.c: Likewise.
	* gcc.target/arm/vselvcsf.c: Likewise.
	* gcc.target/arm/vselvsdf.c: Likewise.
	* gcc.target/arm/vselvssf.c: Likewise.
	* lib/target-supports.exp: Define effective-target arm_cpu_cortex_a57.
	Update effective-target arm_v8_1_lob_ok to use -mcpu=unset.

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