runtime: implement osyield in Go instead of assembly on Windows

runtime: implement osyield in Go instead of assembly on Windows

Windows APIs are normally not arch-specific, so it's better to
implement them in Go instead of assembly.

It was previously implemented in assembly because it was the only way
to support calls without a valid g. This CL defines a new function,
stdcall_no_g, that can be used in such cases.

Change-Id: I26a223b918c6c462b06ac256bdacf9ddb78752bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/526476
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>