Fixed Makefile for -jX and git clean
Various fixes of the GNU Makefile

* The double quote (") does NOT have special meaning
  in the assignments.
* Use := instead of = if we mean it.
* $(HOST_ARCH) for selecting 64 bits toolchains. (provided by NDK r8e)
* Cleanup the messy "cat | sed | sed | sed" commands, replaced
  with a single sed command.