M68k experimental backend support

@mshockwave is a PhD student working on an experimental backend for LLVM targeting Motorola 68000 series CPUs (aka m68k).

Configure and Build

$ cmake ... -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD="M68k;"

GNU binutils dependencies

$ sudo apt install binutils-m68k-linux-gnu

Kernel build

$ ARCH=m68k CROSS_COMPILE=m68k-linux-gnu- make CC=clang -j71 defconfig
$ ARCH=m68k CROSS_COMPILE=m68k-linux-gnu- make CC=clang -j71

Known issues for LLVM M68k

We are using LLVM Bugzilla to track bugs, features and TODOs. Please click on "Browse" > "libraries" > "Backend: M68k" to see all the relevant items.

Currently the issue that is more relevant to clang-built-linux is lacking support on some assembly syntax, given the fact that the backend is using integrated assembler, which is not as mature as existing GNU AS (i.e. m68k-unknown-linux-as).