Enable optional GCC install and GCC improvements

Enable optional GCC install and GCC improvements

 -By adding MKGCC=yes and MKGCCCMDS=yes on the make commandline
   it is now possible to compile and install GCC on the system.

   Before doing this, if you are not using the build.sh script,
   you will need to call the fetch scripts in order to retrieve
   the sources of GCC and its dependencies.

 -Reduce difference with NetBSD share/mk

   Move Minix-specific parameters from bsd.gcc.mk to bsd.own.mk,
   which is anyway patched, so that bsd.gcc.mk is now aligned
   on the NetBSD version.

 -Clean libraries dependencies, compiles stdc++ only if gcc is
   also compiled (it is part of the gcc sources)

 -Correct minix.h header sequence, cleanup spec headers.

 -Fix cross-compilation from a 32bit host targeting MINIX/arm

Change-Id: I1b234af18eed4ab5675188244e931b2a2b7bd943