Fix Makefile: correct emcc flags for latest Emscripten

[slowriot]

Recent versions of emscripten (tested with 3.1.38) produce two separate fatal errors when building:

emcc: error: linker flag ignored during compilation: '--bind' [-Wunused-command-line-argument] [-Werror]

and

emcc: error: .bc output file suffix used without -flto or -emit-llvm. Consider using .o extension since emcc will output an object file, not a bitcode file [-Wemcc] [-Werror]

This pull request removes the --bind flag from compiler options (it's a linker flag only), and adds -emit-llvm to the bitcode generation step.