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=

Committed: http://code.google.com/p/v8/source/detail?r=8705

[fschneider, 2011-07-21 10:59:16]

LGTM.