Bugfix: Fixed execution of host-* tools
Bugfix: Fixed execution of host-* tools

- Execution of host-gcc and other host-* tools failed when SBOX_TOOLS_ROOT
  was set, because do_exec() did not check if the exec mangling code
  disabled all mapping; it tried to start gcc from SBOX_TOOLS_ROOT and not
  from the host (which of course failed, because rest of the universe didn't
  come from SBOX_TOOLS_ROOT)

lbt/scratchbox2,madscientist42/scratchbox2,h113331pp/scratchbox2,loganchien/scratchbox2,BinChengfei/scratchbox2,neeraj9/sbox2,lbt/scratchbox2,madscientist42/scratchbox2,madscientist42/scratchbox2,h113331pp/scratchbox2,madscientist42/scratchbox2,BinChengfei/scratchbox2,OlegGirko/ldbox,BinChengfei/scratchbox2,madscientist42/scratchbox2,lbt/scratchbox2,neeraj9/sbox2,neeraj9/sbox2,loganchien/scratchbox2,loganchien/scratchbox2,OlegGirko/ldbox,h113331pp/scratchbox2,OlegGirko/ldbox,OlegGirko/ldbox,freedesktop-unofficial-mirror/sbox2,ldbox/ldbox,loganchien/scratchbox2,freedesktop-unofficial-mirror/sbox2,loganchien/scratchbox2,BinChengfei/scratchbox2,lbt/scratchbox2,OlegGirko/ldbox,ldbox/ldbox,h113331pp/scratchbox2,lbt/scratchbox2,freedesktop-unofficial-mirror/sbox2,ldbox/ldbox,neeraj9/sbox2,h113331pp/scratchbox2,neeraj9/sbox2,freedesktop-unofficial-mirror/sbox2,BinChengfei/scratchbox2,h113331pp/scratchbox2,ldbox/ldbox,ldbox/ldbox,freedesktop-unofficial-mirror/sbox2,madscientist42/scratchbox2,freedesktop-unofficial-mirror/sbox2,BinChengfei/scratchbox2,loganchien/scratchbox2,ldbox/ldbox,neeraj9/sbox2,OlegGirko/ldbox