Right now CROSS_BINUTILS_PATH is honoured only when XCC is set, i.e. an out-of-tree compiler. Allow CROSS_BINUTILS_PATH to pass through to a -B option also when using the in-tree compiler.

I think this could be named just BINUTILS_PATH, but the existing CROSS_BINUTILS_PATH could as well. Since they have the same meaning I used the existing, if somewhat confusing, name.