testsuite: arm: Use -march=unset for cortex-m55* tests

testsuite: arm: Use -march=unset for cortex-m55* tests

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

gcc/testsuite/ChangeLog:

	* gcc.target/arm/cortex-m55-nodsp-flag-hard.c: Added option
	"-march=unset".
	* gcc.target/arm/cortex-m55-nodsp-flag-softfp.c: Likewise.
	* gcc.target/arm/cortex-m55-nodsp-nofp-flag-softfp.c: Likesie.
	* gcc.target/arm/cortex-m55-nofp-flag-hard.c: Likewise.
	* gcc.target/arm/cortex-m55-nofp-flag-softfp.c: Likewise.
	* gcc.target/arm/cortex-m55-nofp-nomve-flag-softfp.c: Likewise.
	* gcc.target/arm/cortex-m55-nomve-flag-hard.c: Likewise.
	* gcc.target/arm/cortex-m55-nomve-flag-softfp.c: Likewise.
	* gcc.target/arm/cortex-m55-nomve.fp-flag-hard.c: Likewise.
	* gcc.target/arm/cortex-m55-nomve.fp-flag-softfp.c: Likewise.

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