consolidated writing fields logic and added more builtins for skslc msl backend

Bug: skia:
Change-Id: I6cad948bc68194322f031926ab9a49186ec2302b
Reviewed-on: https://skia-review.googlesource.com/134502
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Timothy Liang <timliang@google.com>
2 files changed