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

[Brooks Davis via llvm-commits]

brooksdavis wrote:

Unfortunately the fix seems to have traded a false negative for a false positive when building compiler-rt on FreeBSD amd64. The 32-bit build then fails with:
```
<...>/llvm-project-19.1.0-rc4.src/compiler-rt/lib/builtins/fp_extend.h:57:9: error: _Float16 is not supported on this target
   57 | typedef _Float16 src_t;
      |         ^
1 error generated.
```

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