The legacy `riscv-none-embed` target triple is incorrect and the
toolchain using it has long been obsolete. With our CI no longer
using the obsolete toolchain, there is no need to handle that one
anymore.
The checks would print their prominently colored warnings and confuse
users, and the comments are more geared to those editing the Makefiles
than to those watching which commands get executed.
Back when specific control of the Rust version used with RIOT was
needed, CARGO_CHANNEL was added to explicitly set the Rust version with
consideration for CI special cases.
Rust's mechansims of selecting a toolchain can be used instead now.
Update the list of target triples to match
`makefiles/arch/riscv.inc.mk`. This fixes compilation with toolchains
other than the obsolete toolchain that uses the incorrect
`riscv-none-embed` triple.
This is also a workaround for Rust's [97685], but primarily to enhance
the error message by pointing out that -Zbuild-std is an option, and
generally presenting the error as RIOT usually does.
[97685]: https://github.com/rust-lang/rust/issues/97685
