This patch removes the GC zone allocator.  It is not used and it
produces several regressions in the testsuite.  Furthermore, it
complicates things for the plan to implement manual GC markers
(http://gcc.gnu.org/wiki/cxx-conversion/gc-alternatives#Do_GC_marking_manually).

Tested on x86_64 with standard checking, --enable-checking=gc and
--enable-checking=release.

From-SVN: r195426