arm: testsuite: improve compatibility of pragma_arch_switch_2.c

This test was explicitly setting the architecture on the command-line and
in the body of the test.  In both cases this causes problems with the auto
FPU setting.  Fix by using the testsuite infrastructure correctly and by
adding +fp to the pragma.

gcc/testsuite:

	* gcc.target/arm/pragma_arch_switch_2.c: Use testsuite infrastructure
	to set the architecture flags.  Add +fp to the pragma that changes the
	architecture.