kselftest/arm64: Set default OUTPUT path when undefined

kselftest/arm64: Set default OUTPUT path when undefined

When running 'make' in tools/testing/selftests/arm64/ without explicitly
setting the OUTPUT variable, the build system will creates test
directories (e.g., /bti) in the root filesystem due to OUTPUT defaulting
to an empty string. This causes unintended pollution of the root directory.

This patch adds proper handling for the OUTPUT variable: Sets OUTPUT
to the current directory (.) if not specified

Signed-off-by: tanze <tanze@kylinos.cn>
Link: https://lore.kernel.org/r/20250515051839.3409658-1-tanze@kylinos.cn
Signed-off-by: Will Deacon <will@kernel.org>