swc-windows-installer.py: Search ProgramW6432 and ProgramFiles(x86)

swc-windows-installer.py: Search ProgramW6432 and ProgramFiles(x86)

It turns out that the 'ProgramFiles' environment variable is set
differently depending on the architecture of process in question.  To
always get the 32-bit x86 version, you should use 'ProgramFiles(x86)'
[1].  To always get the 64-bit amd64 version, you should use
'ProgramW6432' [2].

To be safe, and support both 32- and 64-bit Windows installs, just
search everything that could be relevant.  Prefer 64-bit locations
(when we have any).

[1]: http://technet.microsoft.com/en-us/library/cc749104%28v=ws.10%29.aspx
     Recognized Environment Variables
     ...
     CSIDL_PROGRAM_FILES  Version 5.0. The Program Files folder.
                          A typical path is C:\Program Files.
     ...
     PROGRAMFILES         Same as CSIDL_PROGRAM_FILES.
     PROGRAMFILES(X86)    Refers to the C:\Program Files (x86) folder on
                          64-bit systems.

[2]: http://msdn.microsoft.com/en-us/library/windows/desktop/aa384274%28v=vs.85%29.aspx#Environment_Variables
     WOW64 Implementation Details
     ...
     64-bit process:
        ProgramFiles=%ProgramFiles%
        ProgramW6432=%ProgramFiles%
     32-bit process:
        ProgramFiles=%ProgramFiles(x86)%
        ProgramW6432=%ProgramFiles%

     The ProgramW6432 and CommonProgramW6432 environment variables
     were added starting with Windows 7 and Windows Server 2008 R2.