Remove custom target json in favour of `-Ctarget-cpu`?

[madsmtm]

Perhaps it is possible for -Ctarget-cpu to replace the need for the custom target specs.

See discussion in rust-lang/rust#131171, opening this issue to track it instead.

Part of #471.

[Patryk27]

Status: rust-lang/rust#131651.

[tones111]

The Rust changes have been integrated in #634. I think this issue can be closed.