[CommandLine][Linux] Don't read argv from /proc/self/cmdline.
Instead of reading from `/proc/self/cmdline`, take advantage of the fact
that the initial stack layout is ABI specified, and that we already have
a pointer into it (`environ`). This lets us walk up the stack until we
find `argc`, at which point we also know where `argv` is.
We do this from a static initializer because a `setenv()` or `putenv()`
can change `environ` (if you add a new environment variable), and it's
even permissible to just outright change `environ` yourself too. It
seems reasonable to suggest to people that they shouldn't be doing those
things from a static initializer, and as long as they don't, they won't
run before we've had a chance to find `argv`.
Just in case someone _does_ do this, we also check that `environ` points
into the stack. If it doesn't, they won't get any arguments, so if that
happens, that's a clue that they're messing with `environ` too early.
This works around a problem (#69658) with Docker Desktop 4.25.0 and
Rosetta, wherein we end up with an extra argument visible in
`/proc/self/cmdline`, and also avoids allocating memory for the command
line arguments.
rdar://117963394
