Optimize rebuilds by reducing $(shell) usage

$(shell) isn't particularly fast in Kati, and they have to be executed
both when reading the makefiles and determining whether the ninja file
needs to be regenerated.

Right now, the regen time is mostly hidden because we run them in
parallel. We've also configured it to ignore any commands that contain
"echo", "date", or the output directory. That happens to remove most
commands that contain side effects, so running them in parallel is fine.

But the side effects contain some important things, like the clean up
necessary when switching products. So I'm removing those filters, and
then we'll need to run the shell commands in sequence, since there will
be side-effects. That makes regen take longer though, so use pure-Make
implementations instead of $(shell) where possible.

This set of changes reduces aosp/master aosp_arm64-eng build $(shell)
usage and time by 2/3:

*kati*: func shell time: 3.135095 / 709
*kati*: func shell time: 1.067331 / 236

Bug: 30947985
Test: Manual test lines for math functions
Test: Compare build-aosp_arm64.ninja before/after
Change-Id: I4fc9d6318957992921972994f277c17918e7e1eb