clang: Don't specify FP16 format

Support for 16 bit floats is enabled by default in Clang [1].
The ARM alternative format is not supported, so __fp16 always uses
IEEE 754-2008 format [2].

[1] https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/ARM/ARMAsmPrinter.cpp#L750-L755
[2] https://clang.llvm.org/docs/LanguageExtensions.html#half-precision-floating-point



Fixes: #55562

Signed-off-by: Patryk Duda <pdk@semihalf.com>