This patch adds common infrastructure for fast TLS support and
implementation on win32. More implementations will be added soon.

Fast TLS is controlled by V8_FAST_TLS define which is enabled by
default in our gyp and scons builds. The scons build has
fasttls={on,off} option so that we can see the effects of slow TLS
when needed.

Review URL: http://codereview.chromium.org/6696112

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7375 ce2b1a6d-e550-0410-aec6-3dcde31c8c00