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

The CMake docs state that `check_c_source_compiles()` checks whether the
supplied code "can be compiled as a C source file and linked as an
executable (so it must contain at least a `main()` function)."

https://cmake.org/cmake/help/v3.30/module/CheckCSourceCompiles.html

In practice, this command is a wrapper around `try_compile()`:

- https://gitlab.kitware.com/cmake/cmake/blob/2904ce00d2ed6ad5dac6d3459af62d8223e06ce0/Modules/CheckCSourceCompiles.cmake#L54
- https://gitlab.kitware.com/cmake/cmake/blob/2904ce00d2ed6ad5dac6d3459af62d8223e06ce0/Modules/Internal/CheckSourceCompiles.cmake#L101

When `CMAKE_SOURCE_DIR` is compiler-rt/lib/builtins/,
`CMAKE_TRY_COMPILE_TARGET_TYPE` is set to `STATIC_LIBRARY`, so the
checks for `float16` and `bfloat16` support work as intended in a
Clang + compiler-rt runtime build for example, as it runs CMake
recursively from that directory.

However, when using llvm/ or compiler-rt/ as CMake source directory, as
`CMAKE_TRY_COMPILE_TARGET_TYPE` defaults to `EXECUTABLE`, these checks
will indeed fail if the code doesn't have a `main()` function. This
results in LLVM using x86 SIMD registers when generating calls to
builtins that, with Arch Linux's compiler-rt package for example,
actually use a GPR for their argument or return value as they use
`uint16_t` instead of `_Float16`.

This had been caught in post-commit review:
https://reviews.llvm.org/D145237#4521152. Use of the internal
`CMAKE_C_COMPILER_WORKS` variable is not what hides the issue, however.

PR #69842 tried to fix this by unconditionally setting
`CMAKE_TRY_COMPILE_TARGET_TYPE` to `STATIC_LIBRARY`, but it apparently
caused other issues, so it was reverted. This PR just adds a `main()`
function in the checks, as per the CMake docs.