m68k has a LLVM port already, so Rust can be implemented for that platform.[0] It would be nice to have LLVM backends for alpha, hppa and sh4 - these older architectures tend to be quite simple so a working LLVM has plenty of value as a reference and for educational use.
(LLVM even used to have an in-tree DEC Alpha backend, though that was back in 2011 and not relevant to any version of Rust.)
LLVM is desirable for other reasons (LLVMpipe for example), so investing into an LLVM port is probably a better use of limited resources than improving and maintaining a retargeted rustc for GCC.
no, as all conditional-compiled platform specific code is missing.
So using it with #[no_core] should work (assuming the WIP part of the backend isn't a problem). But beyond that you have to first port libcore (should be doable) and then libstd (quite a bunch of work).
As far as I understand it, the m68k LLVM port is not ABI-compatible with GCC on Linux because the alignment is wrong: https://wiki.debian.org/M68k/Alignment (page says that the LLVM packages are FTBFS because of this)
The principled way to address this would be to define a new ABI suffix for the m68k-unknown-linux-gnu target triple, replacing the existing 'gnu' which would retain GCC compatibility with 2-byte alignment for int.
(LLVM even used to have an in-tree DEC Alpha backend, though that was back in 2011 and not relevant to any version of Rust.)
[0] Looks like there is basic initial support but no 'core' or 'std' builds yet. https://doc.rust-lang.org/rustc/platform-support/m68k-unknow... This should potentially be fixable.