Remove a lot of platform duplication, and simplify the virtual
memory implementation. Also improve readability by avoiding bool
parameters for executability (use a dedicated Executability type
instead).

Get rid of the Isolate::UncheckedCurrent() call in the platform
code, as part of the Isolate TLS cleanup.

Use a dedicated random number generator for the address
randomization, instead of messing with the per-isolate random
number generators.

TEST=cctest/test-virtual-memory
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/23641009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16637 ce2b1a6d-e550-0410-aec6-3dcde31c8c00