Canonicalise Windows target triple spellings

Construct a uniform Windows target triple nomenclature which is congruent to the
Linux counterpart.  The old triples are normalised to the new canonical form.
This cleans up the long-standing issue of odd naming for various Windows
environments.

There are four different environments on Windows:

MSVC: The MS ABI, MSVCRT environment as defined by Microsoft
GNU: The MinGW32/MinGW32-W64 environment which uses MSVCRT and auxiliary libraries
Itanium: The MSVCRT environment + libc++ built with Itanium ABI
Cygnus: The Cygwin environment which uses custom libraries for everything

The following spellings are now written as:

i686-pc-win32 => i686-pc-windows-msvc
i686-pc-mingw32 => i686-pc-windows-gnu
i686-pc-cygwin => i686-pc-windows-cygnus

This should be sufficiently flexible to allow us to target other windows
environments in the future as necessary.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204977 91177308-0d34-0410-b5e6-96231b3b80d8