rust-common: Fix native signature dependency issues

rust-common: Fix native signature dependency issues

The previous changes cause sstatetests.SStateTests.test_sstate_32_64_same_hash
to fail since RUST_XXX_SYS changes depending on the native architecture. This
is correct but these are accounted for in the layout of paths in TMPDIR so
they should be excluded from the task signatures for the correct behaviour.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 2b9bb4c07d0ab53f58cbdd8a96896780a90be9a8)
Signed-off-by: Steve Sakoman <steve@sakoman.com>