multilib.conf: Ensure that RECIPE_SYSROOT is unchanged for native

multilib.conf: Ensure that RECIPE_SYSROOT is unchanged for native

Ensure that RECIPE_SYSROOT is the same for -native recipes whether
multilib.conf is included or not.

Without this change task signatures for -native recipes change when
switching between MACHINEs that require multilib.conf and those that
don't.

This fix was one of the ones suggested by Khem Raj in
http://lists.openembedded.org/pipermail/openembedded-core/2019-December/290303.html

Add test_sstate_multilib_or_not_native_samesigs test case to
sstatetests.py to ensure that this stays fixed.

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>