skslc SPIR-V mod support

Change-Id: I2df761ff0d25712dc48d367585d4b639e46d5d97
Reviewed-on: https://skia-review.googlesource.com/17243
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
1 file changed