1. Add __extendhfdf2/__extendhfsf2 to return an IEEE half converted to IEEE double/single. 2. Add __truncdfhf2/__extendsfhf2 to truncate IEEE double/single into IEEE half. 3. Add __eqhf2/__nehf2 to return 0 if a == b and a,b are not NAN, otherwise return 1. These are needed by x86 _Float16: https://software.intel.com/content/www/us/en/develop/download/intel-avx512-fp16-architecture-specification.html support in GCC.