Add approx_pow/log2/pow2 to SkVM builder

... in support of programs for colorspacexforms

Change-Id: I72ace09f10511ef8994038a4af3feab8bc1a299e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/278466
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
3 files changed