objtool: Fix compiler flags leaking to fixdep in cross-build

After my previous changes to the objtool build, I was able to cross-build it for arm64, but it seems that it worked by chance. Now that the compiler flags for arm64 user-space include -mbranch-protection=standard, cross-building stopped working again as this option is wrongly used when compiling the native fixdep.

fixdep (like other simple 'hostprogs') is compiled with options from KBUILD_HOSTCFLAGS, so we need to clear that variable and not HOSTCFLAGS.