Add support for --bindir and remember the relative locations of bindir and libdir vs prefix (ie: sysroot)

Potential downsides:
 - right now adds a build dependency on `realpath`, which I'm not sure everyone has, to determine relative directories. It's possible we could substitute something else if this matters.

Upsides:
 - `--libdir` now avoids breaking things at runtime if it isn't in one of the expected locations
 - config diff between windows and everyone else is now confined to the configure script.
Hm, the commit adding these checks only says 'install: Support --libdir and --mandir correctly
    
    This adds a hack to rustc to make it find the library directory
    regardless of whether it is named lib/lib64/lib32.
'

I think this is supposed to make the install script 'just work' in the most common cases for overriding --libdir, even though it's broken in the general case (because rustc hardcodes some knowledge of `libdir`).