util: add double_to_float16 helpers

util: add double_to_float16 helpers

We convert from doubles to half by going through float in between, but
as noted in the comment in this commit, that can give wrong results in
some cases.

Add some helpers to ensure correct results based on rounding mode that
will be used in the next commit.

v2: Use fi/di from u_math.h (Ian)

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25281>