Fix linux-kbuild builds of objtool

• Our previous hack to cross-build objtool stopped working due to an upstream change in 6.3. Fix it.
• In 6.2 objtool gained support for powerpc as the target architecture. We always build it to target x86, so OOT module builds for powerpc are now broken. For linux-kbuild, build it for both targets and add a wrapper that runs the right one, like we do for modpost.

This goes on top of !723 (merged).