main/gcc6: first changes for gcc6 cross compilers

* remove everything that causes conflicts when installing gcc6 and
  gcc6-armhf side by side
* fix up the existing "if cross compiling" code path to use the
  /usr/gcc6 prefix
* link binutils binaries to the locations they are expected to be
  by gcc (this took me hours to figure out....) as well as by the
  linux build system with CROSS_COMPILE
* link all gcc binaries to /usr/bin/gcc6-*