configure.ac: support MSYS with newer config.guess

configure.ac: support MSYS with newer config.guess

Upstream commit
http://git.savannah.gnu.org/gitweb/?p=config.git;a=commitdiff;h=f4ebd3ed097771a729b68e688236aea665e7c1f3
makes both i386 and amd64 MSYS (and MSYS2) systems be detected as
*-pc-msys .

With this patch OpenOCD builds without any additional tweaks on MSYS2
with the latest config.guess.

Change-Id: I1ae4154f76125a84078926b425fa989904639ce0
Reported-by: "Stevens, Kelly E. M." <Kelly.Stevens@gtri.gatech.edu>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/3468
Tested-by: jenkins
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>