cmd/internal/obj/arm64, image/gif, runtime, sort: use math/bits to calculate log2
In several places the integer log2 is calculated using loops or similar
mechanisms. math/bits.Len* provide a simpler and more efficient
mechanisms for this.
Annoyingly, every usage has slightly different ideas of what "log2"
means and how non-positive inputs should be handled. I verified the
replacements in each case by comparing the result for inputs from 0
to 1<<16.
Change-Id: Ie962a74674802da363e0038d34c06979ccb41cf3
Reviewed-on: https://go-review.googlesource.com/c/go/+/721880 Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Knyszek <mknyszek@google.com>