Add a common templatized implementation of OS::DumpBacktrace() and OS::StackWalk() to platform-posix.h and use it for Linux, Mac OS X and FreeBSD.

R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/18431004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15644 ce2b1a6d-e550-0410-aec6-3dcde31c8c00