runtime: unconditionally load AddVectoredContinueHandler on Windows

runtime: unconditionally load AddVectoredContinueHandler on Windows

AddVectoredContinueHandler is available since Windows XP [1], there is
no need to check if it is available.

[1]: https://learn.microsoft.com/en-us/windows/win32/api/errhandlingapi/nf-errhandlingapi-addvectoredcontinuehandler

Change-Id: I1ddc3d58b3294d9876620cd46159d9692694b475
Reviewed-on: https://go-review.googlesource.com/c/go/+/532817
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>