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.

gcc/testsuite/gcc.target/arm/optional_thumb-1.c

@@ -1,6 +1,6 @@
 /* { dg-do compile { target { ! default_mode } } } */
 /* { dg-skip-if "-marm/-mthumb/-march/-mcpu given" { *-*-* } { "-marm" "-mthumb" "-march=*" "-mcpu=*" } } */
-/* { dg-options "-march=armv6-m" } */
+/* { dg-options "-march=armv6-m -mfloat-abi=soft" } */
 
 /* Check that -mthumb is not needed when compiling for a Thumb-only target.  */
 

gcc/testsuite/gcc.target/arm/optional_thumb-3.c

@@ -1,7 +1,7 @@
 /* { dg-do compile } */
-/* { dg-require-effective-target arm_cortex_m } */
+/* { dg-require-effective-target arm_arch_v7em_ok } */
 /* { dg-skip-if "-mthumb given" { *-*-* } { "-mthumb" } } */
-/* { dg-options "-marm" } */
+/* { dg-options "-march=armv7e-m+fp -marm" } */
 /* { dg-error "target CPU does not support ARM mode" "missing error with -marm on Thumb-only targets" { target *-*-* } 0 } */
 
 /* Check that -marm gives an error when compiling for a Thumb-only target.  */