[sksl][compute] Enable SPIR-V compute test outputs

* The compute tests now compile to SPIR-V, with a .asm.comp extension.
  Most of the output contains compilation errors which will be addressed
  in follow up CLs.

* Support the GLCompute and LocalSize entry point decorations. The
  LocalSize is hardcoded to (16, 16, 1) until SkSL supports local size
  declarations.

Bug: b/262428625

Change-Id: I08795badd75788e0724f03d77e1da537d07ceb51
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/729097
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Arman Uguray <armansito@google.com>
20 files changed