wireguard: selftests: restore support for ccache

When moving to non-system toolchains, we inadvertantly killed the
ability to use ccache. So instead, build ccache support into the test
harness directly.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>