[3.15] gh-148665: Enable socket.shutdown function for emscripten (GH-148666) (#154319)

Re-enable the autoconf check for socket.shutdown on Emscripten, as some
Emscripten environments are able to provide this call.
(cherry picked from commit 20b7e8e)

Co-authored-by: Gyeongjae Choi <gyeongjae@cloudflare.com>

Author: miss-islington

Misc/NEWS.d/next/Library/2026-04-17-05-58-27.gh-issue-148665.AlUajE.rst

@@ -0,0 +1 @@
+socket.shutdown function is now enabled for Emscripten builds. However, if the runtime does not implement the shutdown syscall, it will show "Function not implemented" error.

Platforms/emscripten/config.site-wasm32-emscripten

@@ -21,9 +21,6 @@ ac_cv_func_eventfd=no
 ac_cv_func_memfd_create=no
 ac_cv_func_prlimit=no
 
-# unsupported syscall, https://github.com/emscripten-core/emscripten/issues/13393
-ac_cv_func_shutdown=no
-
 # The rest is based on pyodide
 # https://github.com/pyodide/pyodide/blob/main/cpython/pyconfig.undefs.h