Fix for gdbjit.

Fix for gdbjit.

[Yang, 2011-09-23 09:23:15]

NewGC broke the compile option "gdbjit=on" due to undefined variable. This is my
shot at fixing it. However, when compiling in debug mode with gdbjit, running

gdb --arg ./d8_g -e "while(true){i = Math.random()}" --gdbjit

breaking into gdb with ^C, gdbjit seems to do its job. But when restarting the
run in gdb once or twice again, gdb crashes...

[Vyacheslav Egorov (Chromium), 2011-09-23 09:27:55]

atm gdbjit is broken because it will not receive code delete events
(reportdeleteifneeded is not called by codespace sweeper). can you fix that as
well?

http://codereview.chromium.org/8017001/diff/1/src/mark-compact.cc
File src/mark-compact.cc (right):

http://codereview.chromium.org/8017001/diff/1/src/mark-compact.cc#newcode490
src/mark-compact.cc:490: gdbjit = true;
lets instead do 

never_compact = true;
sweep_precise_ = true; 

here.