Linux build: handle CONFIG_OBJTOOL_WERROR=y

Linux 5.16 by default fails the build on objtool warnings. We have
known and understood objtool warnings we can't fix without
involving Linux maintainers.

To work around this we introduce an objtool wrapper script which
removes the --Werror flag.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Attila Fülöp <attila@fueloep.org>
Closes #17456