[ci] Add merge_matrices function to amdgpu_family_matrix.

[ScottTodd]

Sending for consideration... I think some other structure in this file would be less error prone than this (see #1097).

I was considering making Windows build/test gfx115x on presubmit while keeping Linux gfx115x on postsubmit as part of #1349.

• Without a change like this, we have to include both platforms in the same category, since the dictionary merging using (amdgpu_family_info_matrix_presubmit | amdgpu_family_info_matrix_postsubmit) would overwrite the gfx115x entry.
• With this change, presubmit and postsubmit can both have entries for a given architecture (e.g. Linux gfx115x postsubmit and Windows gfx115x presubmit).

[ScottTodd]

Merge conflicts, might retry something like this again later