arm: testsuite: avoid problems with -mfpu=auto in attr_thumb-static2.c

This test overrides the architecture, but fails to describe which
floating-point features are needed.  This causes problems if the ABI
requires FP for parameter passing and -mfpu=auto is selected, so ensure
that one is specified.

gcc/testsuite:

	* gcc.target/arm/attr_thumb-static2.c: Add +fp to the -march
	specification.