selftests/arm64: Clean the fp helper libraries

We provide a couple of object files with helpers linked into several of
the test programs, ensure they are cleaned.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220427181954.357975-4-broonie@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>