Improve sampler resolution on Linux.

Improve sampler resolution on Linux.

Instead of relying on itimer signals from kernel, send them
ourselves from a separate thread. This disables an ability
to profile multiple VM threads on Linux, but it anyway doesn't
work on other platforms, so we need a common solution for
it (issue 913 created to track this).

Review URL: http://codereview.chromium.org/4000007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5711 ce2b1a6d-e550-0410-aec6-3dcde31c8c00