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

[Khem Raj via llvm-commits]

kraj wrote:

as mentioned in https://github.com/llvm/llvm-project/pull/83639 please backport it into 18.x as well.

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