Emscripten 4.0.12 introduced breaking change by changing module factory
to always return promise even if `WASM_ASYNC_COMPILATION=0`. To overcome this, build emscripten compiler without MODULARIZE with custom export of `createRevive`.
- Move the emscripten target flags into the target configuration.
- Improve the readability of the Makefile.
Signed-off-by: xermicus <cyrill@parity.io>