Only run Kati twice on a clean build due to clean_steps.mk

Write into a temp file, then use `cmp` to determine whether to update
the actual file. This means that we'll only run Kati twice on a clean
build, since we'll omit the redundant write during the regeneration
check.

Simplify writing using $(file >) instead of $(shell), which doesn't have
character count limitations.

Bug: 35970961
Test: m clean; m -j nothing; m -j nothing; m -j nothing
Test: Ensure clean_steps.mk is equivalent before/after
Change-Id: Id574f416647434ab8d11ed3481da21b55e8797b7