docs: Mention the upstream bugzilla url about the dlltool vs MSVC issue

Signed-off-by: Martin Storsjö <martin@martin.st>

doc/general.texi

@@ -989,6 +989,8 @@ the linker optimization option to be set to
 "References: Keep Unreferenced Data (/OPT:NOREF)", otherwise
 the resulting binaries will fail during runtime. This isn't
 required when using import libraries generated by lib.exe.
+This issue is reported upstream at
+@url{http://sourceware.org/bugzilla/show_bug.cgi?id=12633}.
 
 @subsection Cross compilation for Windows with Linux