Supported in assembler, disassembler, and binary parser.
The validator does not check SPV_AMD_gcn_shader validation rules
beyond parsing the extension.
Adds generic support for generating instruction tables for vendor
extensions.
Adds generic support for extensions the validator should recognize
(but not check) but which aren't derived from the SPIR-V core
grammar file.
Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/594
