[Toybox] Android sitrep

[enh]

i guess now's a good time for another sitrep, since i've been out sick
all week and so had time to poke at various things, and there's been
quite a lot going on:

* after the genuine dd bug found by the continuous tests was fixed,
i've seen no more test failures there.

* md5sum and sha*sum were motivating cases to actually properly
support macOS builds.

  * i now, for my sins, have three different .config files, three
different generated/ directories, and a short script using NOBUILD to
regenerate the generated/ files.

    * the good news about that is that i no longer have any need to
build toys/android/ stuff on the host. would you like me to clean up
some of the compatibility cruft that (iirc) is only there to support
that? if it's not useful to me, i'm assuming it's not useful to
anyone?

    * the bad news that turned into good news was that this
accidentally regressed someone's kernel builds (because AOSP doesn't
need nproc, and i cleaned up the host configs to only include tools
actually used, not everything that goes on the device). i added nproc
back to the linux host toybox, and the person who was affected added a
kernel build to the presubmit check for a toybox prebuilt update. so
if we break kernel builds in some way we won't see it when i sync with
github, but we will see it when the build folks try to build a new
prebuilt. which is still fairly frequently.

      * anecdotally i've heard that tar and xargs (independently) gave
them problems, but i've yet to receive the promised bug reports.

      * it's still not 100% clear to me whether they consider using
toybox for these builds to be deliberate or accidental. it happened by
accident (in search of a hermetic build), but they've kept it for now
so ... deliberate?

  * there's a macOS toybox prebuilt now too, updated whenever the linux one is.

  * i _added_ md5sum and sha*sum which the mac doesn't normally have
(the slightly different md5 and shasum instead).

  * i switched over everything used in the AOSP build that's currently
toybox on linux to being toybox on the mac too (except date and stat).
that's only been in a day, but it hasn't had to be reverted yet.

  * date and stat need a bit of tree cleanup because there are places
that _expect_ differences in behavior.

* unrelated, i built a static toybox binary.

  * the transitive dependencies are pretty gross thanks to the cgroup
stuff. i wonder if anyone uses that enough for it to be really worth
it, or whether we could drop it without anyone noticing?

  * the chips in those devices are old enough that you need a generic
build to not die immediately with SIGILL. (i didn't have any of my
usual tools to work out _what_ the invalid instruction is.)

  * ran tests (with some difficulty) on jellybean, and had about 100
failures out of about 1200 tests. (`adb shell` didn't return exit
status that far back, so this is grep for PASS and FAIL rather than
the usual output from my little test runner.)

  * a bunch of the failures are because the current tz code in libc.a
doesn't support the tz data format back in jellybean. i _think_ kitkat
was the transition, so i'd like to rerun the tests when i've had
chance to plug a kitkat device in at my desk.

  * regardless, it seems like it might be plausibly useful to make a
static toybox available on ci.android.com. don't know whether you'd
want to link to that (which was why i pinged you on that bug about
switching to README.md like all the cool kids).