[skunicode] Allow building more than one backend

Allow building more than one SkUnicode implementation at a time. Also
fix up libgrapheme build to allow this to work.

Change-Id: I5287351a4b0c8ee35290a8d5dbae4ef5a30683f3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/735397
Reviewed-by: Julia Lavrova <jlavrova@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
7 files changed