Refactor common makefile fragments

In preparation for setting up another Kati stage, move some common
settings and tools into a common folder. This way it's a bit easier to
see that they're safe to use, and that they shouldn't depend on anything
outside of the common folder.

Bug: 117463001
Test: build-aosp_arm.ninja is the same before and after
Change-Id: Ief4b75a4dbe45b73ffd03bf32c60695c816d979d