windows: don't install x64 version into the 'program files (x86)' folder

* Update nodemsi.sln and .wixproj to include support for x64 platform
  - Add ProgramFilesFolderId to the DefineConstants property for each
    configuration/platform's property group with the appropriate value
    (ProgramFilesFolder for x86 builds, ProgramFiles64Folder for x64
    builds)

* Update product.wxs:
  - update the Id value for the "Program Files" Directory element to
    use a preprocessor constant.
  - remove hard-coded platform from the Package element.  MSI platform
    will be automatically detected based on MSBuild's Platform property.
    (This was already supported in the Wix MSBuild targets, we just
    weren't taking advantage of it.)

*  Update vcbuild.bat to set MSBuild's Platform property appropriately,
   defaulting to x86 if not explicitly supplied by the user. Note that
   creating an x64 build requires that vcbuild.bat be run from a VS
   64-bit command prompt.

Closes #3312
Closes #3356