ignore "target-cpu=native" when cross compiling with "target"

[KonradHoeffner]

Problem

-Ctarget-cpu=native in rustflags displays a huge amount of warnings when also using the "target" parameter for cross compilation.

Proposed Solution

Ignore the target-cpu=native parameter when the target parameter is also set (i.e. when cross compiling) and write a single warning message instead of multiple pages, like:

'target-cpu=native' is not compatible with target platform of 'aarch64-unknown-linux-gnu' (ignoring features) 

Notes

Please correct me if this is working as intended, here are my assumptions and the use case:

1. Rust programs should be as fast as possible and I'm never giving executables built on my machine to others (there are GitHub actions that create release binaries and Docker images for that) so it seems reasonable to include rustflags = ["-Ctarget-cpu=native"] under the [build] section of ~/.cargo/config.toml.
2. I want to add the ARM (aarch64) platform to a specific Rust application but the development machine runs on the amd64 platform, so to confirm that cargo check works before adding the new platform to the GitHub action, I'm using cargo check --target aarch64-unknown-linux-gnu. However this will output several pages of warnings like this:

'+adx' is not a recognized feature for this target (ignoring feature)
'+avx2' is not a recognized feature for this target (ignoring feature)
'-hreset' is not a recognized feature for this target (ignoring feature)
'+movdiri' is not a recognized feature for this target (ignoring feature)
'-serialize' is not a recognized feature for this target (ignoring feature)
'-sha512' is not a recognized feature for this target (ignoring feature)
'+vpclmulqdq' is not a recognized feature for this target (ignoring feature)
'+avx512vl' is not a recognized feature for this target (ignoring feature)
'-uintr' is not a recognized feature for this target (ignoring feature)
'-cf' is not a recognized feature for this target (ignoring feature)

[epage]

RUSTFLAGS is opaque to cargo so we cannot filter out values dependent on the context.

#12739 is tracking finding cargo-native forms for different RUSTFLAGS which could allow smarter behavior.

[epage]

I could see rustc having a consolidated warning or error when native isn't related to --target