skcms: support two attributes with GCC

Cherry pick of https://review.skia.org/975316

* Support [[gnu::fallthrough]] with GCC
* support [[clang::musttail]] with GCC

GCC 15 will support [[clang::musttail]], so fix the checks to align
with Skia in src/core/SkRasterPipeline.h to use it if available for non-Clang.

Fix up [[gnu::fallthrough]] while here too, as GCC supports that.

Change-Id: Iaea26698a3d65c42d4f6c6783f14fbc54465e872
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/1038876
Auto-Submit: Kaylee Lubick <kjlubick@google.com>
Reviewed-by: Daniel Dilan <danieldilan@google.com>
Commit-Queue: Daniel Dilan <danieldilan@google.com>