Add an OpenGL enum values in spv_target_env

[dneto0]

There's now an extension to GL to allow use of SPIR-V for shader programs:
https://www.opengl.org/registry/specs/ARB/gl_spirv.txt

We probably need to add entries to spv_target_env for any GL-based environments that will use SPIR-V. The gl_spirv extension provides validation rules.

The extension describes validation rules in seciton "A.spv.2" and allows for variations for OpenGL 4.0, 4.1, 4.2, 4.3, 4.5 due to progression of functionality in those specs.

[dneto0]

Fixed with c296701