gcc: Switch binutils to DEPENDS instead of bl3.

Both gcc and binutils ship an ansidecl.h.  Until now this hasn't been a
problem, but the recent binutils 2.39 update ships an ansidecl.h that removes
things like "PTR" which GCC depends on, and it seems as though the binutils
version is being chosen ahead of the in-tree GCC version so the build breaks.

Avoiding buildink3 ensures the headers aren't visible, and the builds are
now progressing further.

(jperkin)