arm: testsuite: improve compatibility of gcc.target/arm/optional_thumb-*.c

These tests deliberately pass invalid option combinations to check
that the compiler is generating the correct diagnostic.  Nevertheless,
we can improve their compatibility with other testsuite options.  For
optional_thumb-1.c we use a soft-float ABI, while for
optional_thumb2.c we use arm_arch_v7em as the target architecture,
then set the architecture manually.

gcc/testsuite:

	* gcc.target/arm/optional_thumb-1.c: Force a soft-float ABI.
	* gcc.target/arm/optional_thumb-3.c: Check for armv7e-m compatibility,
	then set the architecture explicitly.