mesa/glspirv: Add _mesa_spirv_link_shaders() function

mesa/glspirv: Add _mesa_spirv_link_shaders() function

This is the equivalent to link_shaders() from
src/compiler/glsl/linker.cpp, but for SPIR-V programs. It just
creates the program and its gl_linked_shader objects, giving drivers
the opportunity to implement any linking of SPIR-V shaders they choose,
at a later stage.

v2: Bail out if we see more that one shader for the same stage, and
    add a corresponding comment. (Timothy Arceri)

v3:
  * Adds also a linker error log to the condition above, with a
    reference to the specification issue. (Timothy Arceri)
  * Squash with the patch adding the function boilerplate (Timothy
    Arceri)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>