Move git to libexec, remove all binaries other than `julia` from bin directory of install targets

@vtjnash would this screw up anything on the Windows side of things? Specifically, I'm looking at [these lines](https://github.com/JuliaLang/julia/blob/d3a1856d1f87cce3869d15f354bd5c2c801ebbb9/Makefile#L229-233), it seems that perhaps we can even get rid of the removal of the `llvm` binaries, as they shouldn't ever get copied over now.  Without this change, the number of programs placed into `$(PREFIX)/bin` was truly amazing.  As seems to be a typical reaction when we fix bugs like this, "I'm amazed this ever worked at all".  ;)