kselftest/arm64: build BTI tests in output directory

The arm64 BTI selftests are currently built in the source directory,
then the generated binaries are copied to the output directory.
This leaves the object files around in a potentially otherwise pristine
source tree, tainting it for out-of-tree kernel builds.

Prepend $(OUTPUT) to every reference to an object file in the Makefile,
and remove the extra handling and copying. This puts all generated files
under the output directory.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230815145931.2522557-1-andre.przywara@arm.com


Signed-off-by: Will Deacon <will@kernel.org>