devel/spirv-tools: update to 2018.6
Changes: https://github.com/KhronosGroup/SPIRV-Tools/compare/v2018.4...v2018.6
Changes: https://github.com/KhronosGroup/SPIRV-Headers/compare/ff684ff...4618b86e9e4b0
Submitted by: Greg V (maintainer)
Differential Revision: https://reviews.freebsd.org/D18711