lib: Switch to w64 vendor for MinGW

It is needed for the `-municode` flag, supposedly.

lib/systems/examples.nix

@@ -207,7 +207,7 @@ rec {
 
   # 32 bit mingw-w64
   mingw32 = {
-    config = "i686-pc-mingw32";
+    config = "i686-w64-mingw32";
     libc = "msvcrt"; # This distinguishes the mingw (non posix) toolchain
     platform = {};
   };
@@ -215,7 +215,7 @@ rec {
   # 64 bit mingw-w64
   mingwW64 = {
     # That's the triplet they use in the mingw-w64 docs.
-    config = "x86_64-pc-mingw32";
+    config = "x86_64-w64-mingw32";
     libc = "msvcrt"; # This distinguishes the mingw (non posix) toolchain
     platform = {};
   };

lib/systems/parse.nix

@@ -208,6 +208,9 @@ rec {
   vendors = setTypes types.openVendor {
     apple = {};
     pc = {};
+    # Actually matters, unlocking some MinGW-w64-specific options in GCC. See
+    # bottom of https://sourceforge.net/p/mingw-w64/wiki2/Unicode%20apps/
+    w64 = {};
 
     none = {};
     unknown = {};