sstate: reduce 32/64bit host sstate reuse problems

BUILD_MARCH, SITEINFO_ENDIANNESS, and SITEINFO_BITS affected native
signatures, and now changes to native signatures also affect target recipes
that depend upon them. Interestingly, changes to BUILD_ARCH will cause natives
to rebuild, but not the target recipes that depend upon them, as BUILD_ARCH is
dynamically produced with ${@}, and its unexpanded form is what goes into
signatures, but the expanded version ends up in the sstate filename. Since the
other variables should only change for native recipes when BUILD_ARCH does, it
should be safe to exclude them from the signatures, thereby restoring the
ability to reuse target sstate archives between 32 and 64 bit build hosts.

JIRA: SB-2976

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Merge pull request #262 from MentorEmbedded/sb-2976

sstate: reduce 32/64bit host sstate reuse problems