glsl: use linked_shaders bitmask to iterate stages for subroutine fields

glsl: use linked_shaders bitmask to iterate stages for subroutine fields

This should be faster than looping over every stage and null checking, but
will also make the code a bit cleaner when we switch to getting more fields
from gl_program rather than from gl_linked_shader as we can just copy the
pointer and not need to worry about null checking then copying.

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