This fixes an upstream bug introduced in emscripten 1.38.34 that causes dlsym
to always return 0 for function pointers when EMULATE_FUNCTION_POINTER_CASTS is
enabled. This is a simple typo, since `mangled == "_" + symbol`. This piece of
code is no longer present in upstream master, hence there is no effort to
upstream the fix.

This patch is tested in emsdk/tests/test_emulate.py

diff --git a/emsdk/fastcomp/emscripten/src/library.js b/emsdk/fastcomp/emscripten/src/library.js
index 97cca10..5c002d7 100644
--- a/emsdk/fastcomp/emscripten/src/library.js
+++ b/emsdk/fastcomp/emscripten/src/library.js
@@ -2194,7 +2194,7 @@ LibraryManager.library = {
     // can call (which does not use that ABI), as the function pointer would
     // not be usable from wasm. instead, the wasm has exported function pointers
     // for everything we need, with prefix fp$, use those
-    result = lib.module['fp$' + symbol];
+    result = lib.module['fp$' + mangled];
     if (typeof result === 'object') {
       // a breaking change in the wasm spec, globals are now objects
       // https://github.com/WebAssembly/mutable-global/issues/1