config.sub should normalize *-*-windows-*

[Po Lu]

x86_64-pc-windows-* is first and foremost a _misnomer_.  The format of a
configuration triplet (or quadruplet) is set in stone:
MACHINE-VENDOR-[KERNEL-]OS.

Given that the MinGW ABI does not constitute the GNU operating system
executing on the MS-Windows kernel, and MSVC is not an operating system,
such blunders should be ignored, or at least normalized to one of the
existing operating system values: x86_64-pc-mingw* for MinGW, and
x86_64-pc-winnt for MSVC.