Use .dsl.cpp suffix for DSL C++ instead of _dsl.cpp.

This requires the GN processing to be slightly more complex, as GN has
no native support for more than one extension on a file.
Context: https://groups.google.com/a/chromium.org/g/gn-dev/c/RdEpjeYtb-4

Change-Id: I630511fca9eb291f0e414481ef439f18a8e1b72f
Bug: skia:11854
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/396197
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
5 files changed