objtool always uses HOSTCC, HOSTLD, and HOSTAR, so we need to override
these on the command line for cross-builds of linux-kbuild. But it
also builds fixdep which still needs to be native in a cross-build.
Add support for REALHOSTCC and REALHOSTLD variables which, if set,
override HOSTCC and HOSTLD for fixdep only.
Gbp-Pq: Topic debian
Gbp-Pq: Name fixdep-allow-overriding-hostcc-and-hostld.patch