Based on existing softfloat64 support and Berkeley SoftFloat. This is
targeted at drivers that can't preserve denorms, so operations where
denorm support is irrelevant like conversions to/from integers aren't
handled.
Because the existing mechanism used by Gallium for softfloat64 doesn't
support includes, we unfortunately can't extract common code into a
header. This can be done later if we switch Gallium to using glslang and
spirv-to-nir.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37608>
