[compiler-rt] [builtins] Fix missing main() function in float16/bfloat16 support checks (PR #104478)

[via llvm-commits]

overmighty wrote:

There won't be another LLVM 18.x release, but I guess we could backport it into 19.x at least. See https://github.com/llvm/llvm-project/pull/101358#issuecomment-2262585319.

https://github.com/llvm/llvm-project/pull/104478