runtime: add an exit hook facility

runtime: add an exit hook facility

Add a new API (not public/exported) for registering a function with
the runtime that should be called when program execution terminates,
to be used in the new code coverage re-implementation. The API looks
like

  func addExitHook(f func(), runOnNonZeroExit bool)

The first argument is the function to be run, second argument controls
whether the function is invoked even if there is a call to os.Exit
with a non-zero status. Exit hooks are run in reverse order of
registration, e.g. the first hook to be registered will be the last to
run. Exit hook functions are not allowed to panic or to make calls to
os.Exit.

Updates #51430.

Change-Id: I906f8c5184b7c1666f05a62cfc7833bf1a4300c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/354790
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>