Fix duplicated methods for POSIX platforms.

No need to duplicate OS::NumberOfCores() for every POSIX platform.
No need to duplicate OS::Sleep() for every POSIX platform.
No need to duplicate OS::Abort() and OS::DebugBreak() for every POSIX platform.
No need to duplicate OS::Free() for every POSIX platform.
Move #ifdef'd OS::ProtectCode() and OS::Guard() to platform-posix.cc.
No need to duplicate OS::AllocateAlignment() for every POSIX platform.
No need to duplicate OS::PostSetUp() for every POSIX platform.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/20014005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15820 ce2b1a6d-e550-0410-aec6-3dcde31c8c00