runtime: allow builtin write function to be redirected with function pointer

The x/sys/windows package currently uses go:linkname for other facilities
inside of runtime that are not suitable to be exposed as a public API
due to their dangers but are still necessary for manipulating any
low-level plumbing that the runtime controls.

Logging, via the built-in println and panic handler, is one such
low-level plumbing feature. In this case, x/sys/windows/svc needs to be
able to redirect panics to the Windows event log. Because the event log
is a complicated interface, this requires a bit more fiddling than the
simple solution used on Android (baking it into runtime itself), and
because Windows services are very diverse, the event log might not even
always be a desirable destination.

This commit accomplishes this by exposing a function pointer called
"overrideWrite" that low-level runtime packages like x/sys/windows/svc
can use to redirect output logs toward the event log or otherwise.

It is not safe or acceptable to use as a generic mechanism, and for that
reason, we wouldn't want to expose this as a real stable API, similar to
the other instances of go:linkname in x/sys/windows. But for packages
that must interoperate with low-level Go runtime fundamentals, this is a
safety hatch for packages that are developed in tandem with the runtime.
x/sys/windows is one such package.

Updates #42888.

Change-Id: I77a32ff7e1494324e8cc38e792e007f86d32672d
Reviewed-on: https://go-review.googlesource.com/c/go/+/278792


Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>