pw_system: Make blatant that SystemStart() clobbers the stack

On FreeRTOS, the target-specific port may reset the machine stack
pointer register to reclaim the memory that is "wasted" by a call that
never returns. This can be bad if the stack contains C or C++ values
that are referenced by other systems, as effectively those values will
be overwritten. The low level manipulation also means C++ destructors
are not invoked.

It's safe to assume that FreeRTOS is not unique in this regard, so let's
make it blatantly clear that pw::SystemStart() will trash the
existing stack by deprecating the existing function name, and using a
new eye-catching one: "pw::system::StartAndClobberTheStack".

Unfortunately there is no good way to detect this automatically. Some
stack use by the compiler is almost inevitable, and we don't control how
`SystemStart()` is invoked. All we can do it call it out, and give some
guidance on how to best prepare for this happening.

This patch does the rename, documents why, and adjust the examples and
tests to match.

Bug: 382073705

Change-Id: I31dc3800efa269af3cd2a7357df5bd5820a27463
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/335555
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Lloyd Pique <lpique@google.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>