[llvm][docs] Update CMake commands for cross compiling Arm builtins

[DavidSpickett]

This does a few things:

• LLVM_CONFIG_PATH is deprecated, use LLVM_CMAKE_DIR instead.
• Don't use $ before command examples. I would normally, but the key cmake commands didn't use it so I removed it from all commands.
• Makes the commands shown full commands, so you don't have to piece them together.
• Uses shell variables to cut down on repetition and make this easier to port to other targets.
• Adds a few options to disable more compiler-rt things.
• Use the built in cmake options for sysroot and toolchains.
• Include test options in the first cmake command, so you don't have to re-do the whole thing after you read the testing section.
• Removes the section about using BaremetalARM.cmake.

The closest I got to getting that cache to work was:

SYSROOT=/home/david.spickett/arm-gnu-toolchain-14.3.rel1-x86_64-arm-none-eabi/arm-none-eabi/libc
LLVM_TOOLCHAIN=/home/david.spickett/LLVM-20.1.8-Linux-X64/

cmake \
  -G Ninja \
  -DCMAKE_C_COMPILER=${LLVM_TOOLCHAIN}/bin/clang \
  -DBAREMETAL_ARMV6M_SYSROOT=${SYSROOT} \
  -DBAREMETAL_ARMV7M_SYSROOT=${SYSROOT} \
  -DBAREMETAL_ARMV7EM_SYSROOT=${SYSROOT} \
  -DCMAKE_BUILD_TYPE=Release \
  -DLLVM_ENABLE_RUNTIMES="compiler-rt" \
  -C ../llvm-project/clang/cmake/caches/BaremetalARM.cmake \
  -DCOMPILER_RT_BUILD_BUILTINS=ON \
  -DCOMPILER_RT_BUILD_LIBFUZZER=OFF \
  -DCOMPILER_RT_BUILD_MEMPROF=OFF \
  -DCOMPILER_RT_BUILD_PROFILE=OFF \
  -DCOMPILER_RT_BUILD_CTX_PROFILE=OFF \
  -DCOMPILER_RT_BUILD_SANITIZERS=OFF \
  -DCOMPILER_RT_BUILD_XRAY=OFF \
  -DCOMPILER_RT_BUILD_ORC=OFF \
  -DCOMPILER_RT_BUILD_CRT=OFF \
  ../llvm-project/runtimes

All this does is build the x86 builtins. I tried forcing the issue with:

  -DBUILTIN_SUPPORTED_ARCH="armv7m;armv6m;armv7em" \

But again, just x86.

It's probably something deep in compiler-rt failing a compiler check for the Arm targets. Even if that's the case, fixing that means adding more options to the cmake command.

I can't find evidence of a full command using this cache file since the commit that introduced it and that command no longer works.

I think if you ever got this to work again the command would be as long and complex as the ones already shown in the document.

I would also argue that some of the other caches, for example Fuschia's, are much better example of multi-target runtimes builds. If what's in this document isn't enough, folks should be learning from those files and about the runtimes build overall before attempting anything complex (though it does not take much to be "complex").

[DavidSpickett]

I looked at whether it should use the runtimes build and maybe it will eventually but it's easier to not have to explain any of that here. It's still valid, as far as I know, to configure using compiler-rt/.

[efriedma-quic]

Add a variable for <path to arm-none-linux-gnueabihf toolchain>, since you're using it in multiple places?

And maybe a brief explanation of why you need the baremetal toolchain, since it isn't obvious (ref #127227).

[DavidSpickett]

Added the variables.

There's two parts to why we need so many sysroots. The first is needing C library headers at all, so I've added a note at the beginning of the doc saying that.

The second is making sure the toolchain is set up for the correct target so you don't build "bare metal" builtins that actually make some Linux assumption. That I have noted in this section in a new paragraph before the cmake command.

I was actually able to use the A profile toolchain for both builtins (M profile target) and tests (A profile target). I don't want to recommend that though because it seems like a compatibility issue waiting to happen.