Added 64-bit Windows build through SCons.

Added 64-bit Windows build through SCons.

The SCons build now supports building for 64-bit Windows. This still requires that the SCons build is passed an env parameter containing the PATH and LIB for the 64-bit build as SCons autodetects the 32-bit environment.

Lowered the warning level for the 64-bit build temporarily.

Added a verbose option to SCons to display the startup banner for the Microsoft Visual C++ tools.
Review URL: http://codereview.chromium.org/174605

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2774 ce2b1a6d-e550-0410-aec6-3dcde31c8c00