These CLI options are now forwarded to the stage1 backend.

We're not going to support the -mllvm CLI option any longer. As a
compromise, we unconditionally tell LLVM to output intel x86 syntax when
using -femit-asm.

Simplify stage1 logic; it no longer has the concept of an output
directory. --output-dir is no longer a valid CLI option. cmake uses
the `-femit-bin=[path]` option.

Note the changes to test/cli.zig. This breaks the CLI API that Godbolt
is using so we're going to want to open a PR to help them upgrade to the
new CLI for the upcoming Zig 0.7.0 release.