libqt4-script: avoid 2GB VIRT memory usage by JIT

See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=712387 for the original bugreport.
See https://bugs.webkit.org/show_bug.cgi?id=118733 for the webkit bugreport.

I've attached the patch here as well: it uses MAP_32BIT instead of allocating 2GB of VIRT on x86-64, and avoids problems with lack of overcommit/swap.
Otherwise applications like KDE's plasma end up using ~3GB of RAM, and kwin similarly, and you soon end up running out of physical RAM even on a machine with 8GB of physical RAM.
Given that a javascript JIT is not a core feature of a Qt application like KDE, I think it should be more conservative about its memory usage. (in fact I wouldn't mind
having a way to turn off the JIT, but that is another bugreport).