Add base variables for windows clang

This just copies most of what exists for clang on Linux, but doesn't
actually turn on clang support.

Bug: 69933068
Test: m nothing; compare ninja files
Change-Id: I2cf203026e7ced32004a91d7f3baf4a6415085eb

cc/config/clang.go

@@ -77,6 +77,9 @@ var ClangUnknownCflags = sorted([]string{
 	"-fno-inline-functions-called-once",
 	"-mfpmath=sse",
 	"-mbionic",
+
+	// windows
+	"--enable-stdcall-fixup",
 })
 
 var ClangLibToolingUnknownCflags = []string{