Provide a mechinism for a 64-bit client to express its ability to run 32-bit software for Windows

[romw]

In order to do this we need a way for the CC to express alternate platform types to the server scheduler. The agreed upon solution is to include an alternate_platform block with the scheduler request.

Example:

    <platform_name>windows_x86_64</platform_name>
    <alternate_platform>
        <name>windows_intelx86</name>
    </alternate_platform>

At some future point we shuold add additional information like how much slower the alternate platform is from the native platform, or if more that one alternate platform is supported which one is preferrred.