GCC: add missing static constexpr member definition C++14 requires to definition of static constexpr members to emit a linker symbol. Bug: chromium:819294 Change-Id: I06b7f1f7a6260982cf07a71c67295789c1f03084 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2720261 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>