linker: Refactor intra-stage block compatabililty testing

linker: Refactor intra-stage block compatabililty testing

Also slightly change the compatibility test.  Instead of comparing the
offsets of the block variables, compare the packing mode of the blocks.
Ideally we don't want to assign the offsets until a later stage of
linking.

This is put in a new file called link_uniform_blocks.cpp.  Some new
functions related to uniform blocks are going to live in that file as
well.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Carl Worth <cworth@cworth.org>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>