runtime: factor out windows sigtramp

This CL factors out part of the Windows sigtramp implementation, which
was duplicated in all four architectures. The new common code is
implemented in Go rather than in assembly, which will make Windows
error handling easier to reason and maintain.

While here, implement the control flow guard workaround on
windows/386, which almost comes for free.

Change-Id: I0bf38c28c54793225126e161bd95527a62de05e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/458135


Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>