soft-fp: Add __extendhfsf2/__extendhfdf2, __truncsfhf2/__truncdfhf2, __eqhf2/__nehf2

soft-fp: Add __extendhfsf2/__extendhfdf2, __truncsfhf2/__truncdfhf2, __eqhf2/__nehf2

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.