Benchmark improvements (emscripten-core#5798)

* cleanup benchmark code
* add cheerp benchmark target
* add utilities to get binaryen and its bin dir
* get the binaryen port when we actually need it, so we don't fetch it early (which, for the wasm backend, might be during compilation of system libraries in ports)
* add a test for the wasm backend building ok with the binaryen port

Bump version to 1.37.21 to rebuild library_pthread.c for PROXY_TO_PTH…

…READ=1 option.

Bump version to 1.37.20 to pick up new aligned_alloc() to build.

Bump version to 1.37.19 to rebuild cache after previous commit to rem…

…ove atomics exchange polyfill.

Merge pull request emscripten-core#5359 from juj/pthread_stubs

Pthread stubs