Update emscripten

Freetype upstream made changes to remove the need for our fcstat patch,
but we actually also need to update emscripten as linking to fstatfs is
bugged in our currently used version and will lead to runtime errors.

- Since emscripten 2.0.26 unresolved symbols cause errors at link time
  instead of only at runtime.
- The following commit contained in 2.0.27 fixed fstatfs linking:
    emscripten-core/emscripten@2126d3c

Cherry-picked from: libass@f92d7ce

Dockerfile

@@ -1,4 +1,4 @@
-FROM emscripten/emsdk:2.0.20
+FROM docker.io/emscripten/emsdk:2.0.34
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
         build-essential \