stdlib: link against legacy_stdio_definitions on Windows

Because Microsoft inlines the definitions of the printf family of
functions, we end up with undefined references to them.  Add an explicit
request to link against the `legacy_stdio_definitions` library when
`ucrt.C.stdio` is used which provides out-of-line definitions for them.
This fixes the building of the `stdlib/VarArgs' test.

Author: compnerd

stdlib/public/Platform/ucrt.modulemap

@@ -60,6 +60,13 @@ module ucrt [system] {
     module stdio {
       header "stdio.h"
       export *
+
+      // NOTE(compnerd) this is a horrible workaround for the fact that
+      // Microsoft has fully inlined nearly all of the printf family of
+      // functions in the headers which results in the definitions being elided
+      // resulting in undefined symbols.  The legacy_stdio_definitions provides
+      // out-of-line definitions for these functions.
+      link "legacy_stdio_definitions"
     }
 
     module stdlib {