[libc] Implement log10f correctly rounded for all rounding modes.

[lntue]

Summary

Based on RLIBM implementation similar to logf and log2f. Most of the exceptional inputs are the exact powers of 10.

[lntue]

Fix comments.

[sivachandra]

LGTM but please wait for approval from others.

libc/src/math/generic/log10f.cpp

135

Nit: Does it make sense to add a comment telling the reader about the decimal value?

libc/test/src/math/log10f_test.cpp

36

Nit: Can you add a comment telling the reader what these numbers correspond to?

[zimmermann6]

this revision passes all exhaustive tests, for the four rounding modes, great work!

[santoshn]

Looks good to me. Thanks.

[lntue]

Add decimals of exceptional values to comments.