Sort $(MXE_TARGET_LIST) by triplets instead of lib types

Nothing in the Makefile is affected else than the generation of default
settings.mk.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Add build matrix generation target

Based on a patch by Tobias Gruetzmacher <tobias-git@23.gs> (@TobiX).

Signed-off-by: Timothy Gu <timothygu99@gmail.com>