Don't build GDB in canadian build->target

In canadian build->target is built first (eg. in order to build/run
executable on build system to generate table), then host->target.

Don't build GDB in build->target, because GDB doens't need it.  Besides,
--with-python= pointing to "host" header can fail "build"'s gcc.

Change-Id: Ic1a8a9d6fe5974a6747ca4342a336beea9987f47