Re-apply: Implement mapping randomization for 64-bit Linux.

Re-apply: Implement mapping randomization for 64-bit Linux.

- 32-bit Linux already seems to scatter the mmap() chunks around;
  64-bit didn't.

- Seed the system random number generator a little better (we needlessly trunctaed microsecond resolution to millisecond resolution).

- Will automatically take advantage of better entropy when V8::RandomPrivate uses it.

R=fschneider@chromium.org,cevans@chromium.org
BUG=v8:805
TEST=

Review URL: http://codereview.chromium.org/7464022
Patch from Chris Evans <cevans@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8705 ce2b1a6d-e550-0410-aec6-3dcde31c8c00