selftests/arm64/fp: Update Makefile for out of tree builds

Previously the Makefile dependency tracking did not work as expected
with out of tree builds leading to targets requiring the linking of
multiple object files to be missed by the selftest build system. This
patch makes use of the lib.mk all target instead of defining our own and
also ensures that dependencies are correct by prepending '$(OUTPUT)' to
output files.

Note that this change is picked from Linux 6.1 but with additional
functionality added by subsequent commits removed; this change only aims
to fix the build system errors.

Signed-off-by: Jackson Cooper-Driver <jackson.cooper-driver@arm.com>