compiler-rt: Fix FLOAT16 feature detection

CMAKE_TRY_COMPILE_TARGET_TYPE defaults to EXECUTABLE, which causes
any feature detection code snippet without a main function to fail,
so we need to make sure it gets explicitly set to STATIC_LIBRARY.

Bug: https://github.com/ROCm/rocFFT/issues/439
Bug: https://github.com/ROCm/rocBLAS/issues/1350
Bug: https://bugs.gentoo.org/916069
Closes: https://github.com/llvm/llvm-project/pull/69842

Reviewed by: thesamesam, mgorny