The coconut-svsm project which provides a low level firmware for AMD
SEV-SNP virtual machines uses 'x86_64-unknown-none' as its build
target and has recently removed the requirement to use rust nightly[1].
Thus adding 'x86_64-unknown-none' as a build target will enable us to
build coconut-svsm in Fedora using the standard Rust toolchain packages.
[1] https://github.com/coconut-svsm/svsm/pull/81
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>