arm: Fix testisms introduced with fix for pr target/95646

arm: Fix testisms introduced with fix for pr target/95646

This patch changes the test to use the effective-target machinery disables the
error message "ARMv8-M Security Extensions incompatible with selected FPU" when
-mfloat-abi=soft.
Further changes 'asm' to '__asm__' to avoid failures with '-std=' options.

gcc/ChangeLog:
2020-07-06  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* config/arm/arm.c (arm_options_perform_arch_sanity_checks): Do not
	check +D32 for CMSE if -mfloat-abi=soft

gcc/testsuite/ChangeLog:
2020-07-06  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* gcc.target/arm/pr95646.c: Fix testism.