Provide target-cpu and target-features to buildscripts

[lukaslueg]

As far as I can see there is no reliable way for a buildscript to determine the value of target-cpu and target-features. Could we add and env-flag for Cargo to provide the values that will be used when the final crate is compiled for the target platform?

Downstream feature request from here

[ehuss]

Target features should be exposed in the CARGO_CFG_TARGET_FEATURE environment variable. The request to expose the target-cpu is tracked in rust-lang/rust#44036.

[ehuss]

Going to close since supporting target-cpu is tracked in rustc.