mesa/glsl: add bitmask to track stages a program was linked against

mesa/glsl: add bitmask to track stages a program was linked against

This will be used to enable us to store the current gl_program
rather than gl_shader_program in the gl_pipline_object allowing
us to simplify handing of validation.

Also we should not be depending on _LinkedShader for this information
as it may contain shaders from a failed linking attempt rather than
the current program still in use.

We could also use this mask to iterate over the stages during linking
with _mesa_bit_scan() rather then the current method of NULL checking
each stage.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>