nir/spirv: Add defaults for GS input/output primitive types
nir/spirv: Add defaults for GS input/output primitive types

These are supposed to be specified in the SPIR-V source as SpvExecutionMode
enums but glslang isn't giving them to us.  A bug has been filed:

https://github.com/KhronosGroup/glslang/issues/84

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler