[llvm-branch-commits] [compiler-rt] release/19.x: [builtins] Fix missing main() function in float16/bfloat16 support checks (#104478) (PR #106843)

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overmighty wrote:

> @overmighty (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix. When you are done, please add the release:note label to this PR.

Fixed compiler-rt builtins' CMake checks for `_Float16` and `__bf16` support incorrectly failing depending on the directory used as CMake source directory. This resulted in `__bf16` builtins and some `_Float16` builtins not being built, as well as `_Float16` builtins falling back to `uint16_t` and thus not using the same register as LLVM expects for their argument or return value.

https://github.com/llvm/llvm-project/pull/106843