make constexpr variants for SkCLZ/SkCLT and similar functions

Make a parallel constexpr set of functions for bit log and power
functions. These are labeled as *_portable.

Change-Id: I40cca37eb3e75c496638d8b58d622467dd692b27
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/333657
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
1 file changed