testsuite: [arm] Add arm_softfp_ok and arm_hard_ok effective targets.

testsuite: [arm] Add arm_softfp_ok and arm_hard_ok effective targets.

For arm-linux-gnueabi* targets, a toolchain cannot support the
float-abi opposite to the one it has been configured for: since glibc
does not support such multilibs, we end up lacking gnu/stubs-*.h when
including stdint.h for instance.

This patch introduces two new effective targets to detect whether we
can compile tests with -mfloat-abi=softfp or -mfloat-abi=hard.

This enables to make such tests unsupported rather than fail.

2020-04-22  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/testsuite/
	* lib/target-supports.exp (arm_softfp_ok): New effective target.
	(arm_hard_ok): Likewise.

	gcc/
	* doc/sourcebuild.texi (arm_softfp_ok, arm_hard_ok): Document.