Fix size of store buffer to depend on pointer size.

Fix size of store buffer to depend on pointer size.

This makes the store buffer size depend on the pointer size so that
store buffers can hold the same number of entries on all architectures.
The number of pages marked scan-on-scavenge should decrease on x64 and
performance of splay should drastically improve on x64.

R=erik.corry@gmail.com

Review URL: http://codereview.chromium.org/8498013

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9922 ce2b1a6d-e550-0410-aec6-3dcde31c8c00