Disable regression test for issue #18804 on Emscripten and Asmjs

The Emscripten compiler does not support weak symbols at the moment.

Signed-off-by: Gabriel Smith <ga29smith@gmail.com>

src/test/run-pass/issue-18804/auxiliary/lib.rs

@@ -8,6 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// ignore-asmjs no weak symbol support
+// ignore-emscripten no weak symbol support
+
 #![crate_type = "rlib"]
 #![feature(linkage)]