[arm] Make gcc.target/arm/copysign_softfloat_1.c more robust

[arm] Make gcc.target/arm/copysign_softfloat_1.c more robust

This test has needlessly restrictive requirements. It tries to force a soft-float target and tries to run.
This makes it unsupportable for any non-soft-float variant.
In fact, the test can be a run-time test for any target, and only the scan-assembler tests are specific to
-mfloat-abi=soft. So this patch makes the test always runnable and makes the scan-assembler checks predicable
on the the new arm_sotftfloat effective target check.

	* doc/sourcebuild.texi (arm_softfloat): Document.

	* lib/target-supports.exp (check_effective_target_arm_softfloat):
	New procedure.
	* gcc.target/arm/copysign_softfloat_1.c: Allow running everywhere.
	Adjust scan-assembler checks for soft-float.

From-SVN: r256940