Judge x86_64 by uname command on a Unix.

I solved a problem which happens when trying to share vimproc between a 32bit Unix machine and a 64bit one: one of thier vimproc_unix.so-s is made incompatible with another.

To solve, I copied make_unix.mak into make_unix32.mak and make_unix64.mak.
Then, define vimproc#util#is_unix64() function and modify the logic to set default value of g:vimproc_dll_path with the function.

If you like these commits, please merge!