syscall: make windows Exit call runtime.exit

syscall: make windows Exit call runtime.exit

Both runtime.exit and syscall.Exit call Windows ExitProcess.
But recently (CL 34616) runtime.exit was changed to ignore
Windows CreateThread errors if ExitProcess is called.

This CL adjusts syscall.Exit to do the same.

Fixes #18253 (maybe)

Change-Id: I6496c31b01e7c7d73b69c0b2ae33ed7fbe06736b
Reviewed-on: https://go-review.googlesource.com/45115
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Austin Clements <austin@google.com>