linux-kbuild: Fix cross-build regression in objtool in 6.3

Upstream commit cd955bdd6aa5 "objtool: Fix HOSTCC flag usage" changed
objtool to use the HOST* variables consistently.  We therefore need to
override those variables when cross-building linux-kbuild.

However, the objtool build also builds and runs fixdep, so that still
needs to be native.  Add and use REALHOSTCC and REALHOSTLD variables
that override HOSTCC and HOSTLD for fixdep only.