[WebAssembly] Strip TLS when "atomics" is not enabled

With f3b4f990, the exclusive source of
truth for whether threads are supported is the -matomics flag.
Accordingly, strip TLS flags when -matomic is not specified, even if
bulk-memory is specified and it would theoretically be supportable.
This allows the backend to compile TLS variables when -mbulk-memory is
enabled but threads are not enabled.

Differential Revision: https://reviews.llvm.org/D125730