glsl: Change some linker loops to use MESA_SHADER_FRAGMENT as a bound.

Linker loops that iterate through all the stages in the pipeline need
to use MESA_SHADER_FRAGMENT as a bound, so that we can add an
additional MESA_SHADER_COMPUTE stage, without it being erroneously
included in the pipeline.
mesa: Add a MESA_SHADER_COMPUTE stage.
squash! i965/gen7: Allow ARB_compute_shader to be enabled via env var.

Not just gen7