Implement C23 log2p1

[Joseph Myers]

On Wed, 10 Apr 2024, Paul Zimmermann wrote:

>        Dear Joseph,
> 
> I tried your patch on a x86_64, it applies cleanly, everything compiles
> cleanly, and I can use the new log2p1 function in both binary32, binary64,
> double-extended and binary128.
> 
> As for accuracy, here are the largest errors I found for rounding to nearest:

In general I encourage adding extra tests to auto-libm-test-in where they 
show errors, for any implementation in glibc, larger than those inputs 
already present (but not so large that they cannot be entered in 
libm-test-ulps files).  So adding such inputs for log2p1 would be 
appropriate after this patch is in, and the same applies for all the other 
C23 functions as they get added.

-- 
Joseph S. Myers
josmyers@redhat.com