genksyms: allow to ignore symbol checksum changes

This adds an "override" keyword for use in *.symvers / *.symref files.
When a symbol is overridden, the symbol's old definition will be used for
computing checksums instead of the new one, preserving the previous
checksum.  (Genksyms will still warn about the change.)

This is meant to allow distributions to hide minor actual as well as fake
ABI changes.  (For example, when extra type information becomes available
because additional headers are included, this may change checksums even
though none of the types used have actully changed.)

This approach also allows to get rid of "#ifdef __GENKSYMS__" hacks in the
code, which are currently used in some vendor kernels to work around
checksum changes.

Signed-off-by: Andreas Gruenbacher <agruen@suse.de>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>