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.