make check failure on mainline CVS

[Graham Stott]

Andreas,

Investigating further I've found that the problem is that the GC collector
kicks in when the entries on the pending_attributes tree-list as returned
by get_pending_attributes are not accessable via any gc roots hence they
are subject to collection by GC.

A quick hack would be to push and pop the GC context at the begining and
end of the  build_unit_elab routine. Such a hack does indeed fix the
failure.

Graham
ps. the GC collector got invoked during a call to expand_expr_stmt within
build_uint_elab.