[WebAssembly] Add support for -mexec-model=reactor

This adds a -mexec-model= command-line flag. The default value is "command"
which is no change from the current behavior. The other option is "reactor"
which enables the WASI Reactor ABI:

https://github.com/WebAssembly/WASI/blob/master/design/application-abi.md

Differential Revision: https://reviews.llvm.org/D62922