[m-rev.] for review: Update README.MinGW regarding MSYS2 and x86_64.

[Peter Wang]

On Fri, 28 Aug 2015 15:43:46 +1000 (AEST), Julien Fischer <jfischer at opturion.com> wrote:
> 
> One of the effects of the those versions is that the string returned by
> config.guess differs:
> 
>      MinGW-w32 Win32 Shell ==> x86_64-pc-mingw32
>      MinGW-w64 Win64 Shell ==> x86_64-pc-mingw64
>      MSYS2                 ==> x86_64-pc-msys

I reworded the part about passing --host to configure.  It might only be
truly necessary if using the original MSYS or MSYS2 shell with MinGW-w64.
(Why not just use the shortcuts above?)

> I suggest that you update all of them (top-level directory, boehm_gc
> and boehm_gc/libatomic_ops) to the latest version.

Done.

Peter