[compiler-rt] [llvm] [compiler-rt] Add check-builtins target for LLVM_ENABLE_RUNTIMES builds (PR #166837)

[Andrew Haberlandt via llvm-commits]

ndrewh wrote:

> Builtins are somehow being tested incorrectly under this PR (that would be bad!)

The problem was that `COMPILER_RT_HAS_${arch}_FLOAT16` was depended upon by `test/builtins/CMakeLists.txt` but was only defined in `lib/builtins/CMakeLists.txt`. Moving it to the shared `builtin-config-ix.cmake` fixes the issue.

https://github.com/llvm/llvm-project/pull/166837