runtime: don't use systemstack for BeforeFork/AfterFork

In https://golang.org/cl/140930043 syscall.BeforeFork was changed to
call beforefork via onM. This was done because at the time BeforeFork
was written in C but was called from Go. While the runtime was being
converted to Go, calls to complex C functions used onM to ensure that
enough stack space was available.

In https://golang.org/cl/172260043 the syscall.BeforeFork and
beforefork functions were rewritten into Go. In this rewrite
syscall.BeforeFork continue to call beforefork via onM, although
because both functions were now in Go that was no longer necessary.

In https://golang.org/cl/174950043 onM was renamed to systemstack,
producing essentially the code we have today.

Therefore, the use of systemstack in syscall.BeforeFork (and
syscall.AfterFork) is a historical relic.  Remove it.

Change-Id: Ia570f556b20e8405afa6c5e707bd6f4ad18fd7ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/341335


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>