Switch to Red Hat's EMSDK in Envoy

We are currently using a hack to compile WASM extensions that is to patch build scripts to use the host's clang in emsdk.

The llvm team is working on productize emsdk, which should eliminate the need of this hack, if we switch to this version of emsdk.

We should work with them to make the necessary changes and tests in order to get rid of this hack.

Acceptance Criteria:

• Figure out which emsdk version we'll need for the 2.2 envoy builds
• Work with peers (Timm Baeder, Tom Stellard, Matt Newsome) to get a product build of the emsdk with the right version
• Remove the hack