[llvm][docs] Update CMake commands for cross compiling Arm builtins (#151544)

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").