Introduce a special CPU for Windows and add msys compiler toolchain.

We might revisit what the default toolchain for Windows should be, but
this CL uses msys to simplify bootstrapping.

--
MOS_MIGRATED_REVID=113665255
diff --git a/src/BUILD b/src/BUILD
index 48afe1a..9741c3d 100644
--- a/src/BUILD
+++ b/src/BUILD
@@ -8,6 +8,7 @@
     srcs = select({
         ":darwin": ["//src/main/native:libunix.dylib"],
         ":darwin_x86_64": ["//src/main/native:libunix.dylib"],
+        ":windows": [],
         "//conditions:default": ["//src/main/native:libunix.so"],
     }),
     visibility = [
@@ -186,6 +187,6 @@
 
 config_setting(
     name = "windows",
-    values = {"compiler": "windows_msys64"},
+    values = {"cpu": "x64_windows"},
     visibility = ["//visibility:public"],
 )