Don't depend upon `touch` executable

Currently GHC depends upon the POSIX touch command for modifying the mtime of files. This incurs a performance tax on some platforms (e.g. Windows) and, more importantly, a significant amount of complexity since we must:

configure the path to touch and propagate this information through the settings file
provide an implementation of touch (named touchy) on Windows

All of this complexity adds a few hundred lines to GHC which can easily be replaced by a five line Haskell implementation. I suggest that we do so.

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