Don't assume that we use LLVM's ld(1) if we use clang(1) as main system

Don't assume that we use LLVM's ld(1) if we use clang(1) as main system
compiler.  Only handle this case when LLD was actually installed during
the cross-compiler setup.  Unbreaks armv7 after switching to clang since
it uses binutils' ld(1).