base_rules.mk optimizations

A few more misc improvements that I found while analyzing the
performance of base_rules.mk.

This brings an aosp-master/aosp_crosshatch-userdebug kati run from 33.3s
to 28.6s

Bug: 158488548
Test: build-aosp_crosshatch.ninja is the same before/after
Change-Id: If99c31cc7b5d7133d70eb644c6095f19060b71e5