wasm32-emscripten: add initial support

This MR adds initial support for the "wasm32-*-emscripten" triplet

contrib/src/ffmpeg/0001-ffmpeg-add-target_os-support-for-emscripten.patch

+From 4bb53ad8e35f899f14e6d6144e60d8caa5c0651c Mon Sep 17 00:00:00 2001
+From: Mehdi Sabwat <mehdi@videolabs.io>
+Date: Mon, 6 Sep 2021 11:49:03 +0200
+Subject: [PATCH 1/1] ffmpeg: add target_os support for emscripten
+
+---
+ configure | 2 ++
+ 1 file changed, 2 insertions(+)
+
+diff --git a/configure b/configure
+index d7a3f50..57ca005 100755
+--- a/configure
++++ b/configure
+@@ -5579,6 +5579,8 @@ case $target_os in
+         ;;
+     minix)
+         ;;
++    emscripten)
++	;;
+     none)
+         ;;
+     *)
+-- 
+2.33.0
+