Test & implement exponential intrinsics

Bug: skia:10913
Change-Id: Id9f2b14ca7443b3375036d588849bc9a20d76e40
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/334156
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
7 files changed