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>

Documentation/rust/arch-support.rst

@@ -17,3 +17,5 @@ Architecture  Level of support  Constraints
 ============  ================  ==============================================
 ``x86``       Maintained        ``x86_64`` only.
 ============  ================  ==============================================
+``um``        Maintained        ``x86_64`` only.
+============  ================  ==============================================

arch/um/Kconfig

@@ -28,6 +28,7 @@ config UML
 	select TRACE_IRQFLAGS_SUPPORT
 	select TTY # Needed for line.c
 	select HAVE_ARCH_VMAP_STACK
+	select HAVE_RUST			if X86_64
 
 config MMU
 	bool