Bug 1713081 - Fix to use the default clang from upstream for SM WASI. r=glandium

To compile SM to .wasm we need to use clang from wasi-sdk, this patch
allows us to use default clang-12 from mozilla's repo.
We remove usage of rpath-link flag because wasm-ld doesn't support it,
enable single thread model because wasi is single threaded
and finally add <unistd.h> becase it is forbidden to use read/write/lseek
without this header.

Differential Revision: https://phabricator.services.mozilla.com/D116207