[Feature Request] Expose arguments for Mojo build target options.

[owenhilyard]

Review Mojo's priorities

• I have read the roadmap and priorities and I believe this request falls within the priorities.

What is your request?

Expose the arguments for the following build flags:

• --target-triple
• --target-cpu
• --target-features
• --march
• --mcpu
• --mtune

For example, include flags to dump all target triples, all features supported for the selected architecture, and the known CPUs.

What is your motivation for this change?

While we can tell people to go look at the clang docs, we don't know what targets Mojo's copy of LLVM has enabled. For example, if amd64-unknown-linux-musl or the mingw windows targets exist, or whether Mojo supports the amx-transpose feature. This is also helpful for stdlib developers because ideally every single CPU feature should be exposed in sys.info. It also helps people with very new hardware determine if the current stable Mojo release supports their hardware.

Any other details?

If there's a way to make the output structured (json or otherwise), we might be able to automatically generate parts of sys.info instead of hand-writing them.