arm: testsuite: improve compatibility of arm/pr78353-*.c

Again, use the infrastructure available to improve the compatibility
of these tests.

gcc/testsuite:

	* gcc.target/arm/pr78353-1.c: Use dg-add-options to manage target
	flags.
	* gcc.target/arm/pr78353-2.c: Likewise.

gcc/testsuite/gcc.target/arm/pr78353-1.c

 /* { dg-do link }  */
 /* { dg-require-effective-target arm_arch_v7a_multilib } */
-/* { dg-options "-march=armv7-a -mthumb -O2 -flto -Wa,-mimplicit-it=always" }  */
+/* { dg-options "-mthumb -O2 -flto -Wa,-mimplicit-it=always" }  */
+/* { dg-add-options arm_arch_v7a } */
 
 int main(int x)
 {

gcc/testsuite/gcc.target/arm/pr78353-2.c

 /* { dg-do link }  */
 /* { dg-require-effective-target arm_arch_v7a_multilib } */
-/* { dg-options "-march=armv7-a -mthumb -O2 -flto -Wa,-mimplicit-it=always,-mthumb" }  */
+/* { dg-options "-mthumb -O2 -flto -Wa,-mimplicit-it=always,-mthumb" }  */
+/* { dg-add-options arm_arch_v7a } */
 
 int main(int x)
 {