Use correct host triple

[catamorphism]

FIXME in driver::driver::host_triple says:

    // FIXME: Instead of grabbing the host triple we really should be
    // grabbing (at compile time) the target triple that this rustc is
    // built with and calling that (at runtime) the host triple.

[brson]

After running into this comment I'm surprised that this hasn't caused any noticeable fallout. The result of this should be that our i386 compilers think that they are x86_64 compilers.

[graydon]

this comment is still present, but the logic has changed; it is now grabbing the build triple from the compilation environment, which might well be correct. I suspect all this needs is some cleanup and clarification (possibly renaming the method)

[alexcrichton]

I believe that this can be closed now. I tried to clarify as well where necessary. If anyone feels that it's wrong to close, feel free to reopen!