Allow build-time customization of the max semispace size. Building a
version of V8 with snapshots and with a non-default max semi-space
size is much easier when you can set the max semispace size in the
build environment.

Review URL: http://codereview.chromium.org/4937001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5823 ce2b1a6d-e550-0410-aec6-3dcde31c8c00