rust: arch/um: Add support for CONFIG_RUST under x86_64 UML

CONFIG_RUST currently supports x86_64, but does not support it under
UML. With the previous patches applied, adding support is trivial:
add CONFIG_HAVE_RUST to UML if X86_64 is set.

The scripts/generate_rust_target.rs file already checks for
CONFIG_X86_64, not CONFIG_X86, so is prepared for UML support.

The Rust support does not currently support X86_32.

Also, update the Rust architecture support documentation to not that
this is being maintained: I intend to look after this as best I can.

Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Sergio González Collado <sergio.collado@gmail.com>
Tested-by: Sergio González Collado <sergio.collado@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>