[testsuite] fix to check_effective_target_arm_hard_vfp_ok

[Janis Johnson <janis_johnson at mentor dot com>]

Tests in gcc.target/arm/aapcs check for floating-point arguments being
passed correctly, but the added flag "-mfloat-abi=hard" can be
overridden by another value in flags used for all tests (like multilib
flags), causing the tests to fail.  The tests in that directory are all
gated on effective-target arm_hard_vfp_ok; this patch changes its code
to return 0 if the test will use a different value of -mfloat-abi.

Tested on arm-none-eabi with a variety of test flags.  I'm hoping to
hear from ARM experts that this is OK; there are a handful of tests in
arm/aapcs that pass even with -mfloat-abi=softfp, so a better solution
might be to skip individual tests for that option.

Janis

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