x86: introduce .hfloat directive

x86: introduce .hfloat directive

This is to be able to generate data passed to {,V}CVTPH2PS and acted
upon by AVX512-FP16 insns. To be able to also use the hex forms
supported for other floating point formats, a small addition to the
generic hex_float() is needed.

Extend existing x86 testcases.