selftests/arm64: Fix O= builds for the floating point tests

Currently the arm64 floating point tests don't support out of tree builds
due to two quirks of the kselftest build system. One is that when building
a program from multiple files we shouldn't separately compile the main
program to an object file as that will result in the pattern rule not
matching when adjusted for the output directory. The other is that we also
need to include $(OUTPUT) in the names of the binaries when specifying the
dependencies in order to ensure that they get picked up with O=.

Rewrite the dependencies for the executables to fix these issues. The
kselftest build system will ensure OUTPUT is always defined.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220427181954.357975-5-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>