libm: Add fmaf16

[tgross35]

Recreated from rust-lang/libm#419

[tmvkrpxl0]

Is it really a placeholder if it (now) has implementation?

[tgross35]

Indeed, updated to a doc comment :)

[tgross35]

Baseline at 1909bf7:

icount::icount_bench_math_group::icount_bench_fmaf16 logspace:(setup_fmaf16())
  Baselines:                   arch_enabled|arch_enabled (old)
  Instructions:                       64543|N/A                  (*********)
  L1 Hits:                            71915|N/A                  (*********)
  LL Hits:                                3|N/A                  (*********)
  RAM Hits:                              18|N/A                  (*********)
  Total read+write:                   71936|N/A                  (*********)
  Estimated Cycles:                   72560|N/A                  (*********)

[quaternic]

At this point we have the result as a sign-magnitude fixed-point value that just needs rounding. The rest of this function might be a useful to factor out, and test independently. Something like

/// Rounds the value `n * 2^s` to `f16`
fn f16_from_u64_with_scale(n: u64, s: i32) -> f16

And this could return

f16_from_u64_with_scale(r64, -40).copysign(/* from rneg */)

View changes since the review

[quaternic]

A handy trick for doing the rounding: The effect of rounding a finite magnitude up is always just +1 to the bits of the float. If the mantissa had its maximum value, it carries into the exponent field but that still gives the correct value because the mantissa is then all zeroes.

View changes since the review