syscall: fix windows WSASendto -d=checkptr violation

WSASendto converts unsafe.Pointer to *syscall.RawSockaddrAny. But that
violates every rule of

https://golang.org/pkg/unsafe/#Pointer

Implement WSASendto by calling Windows WSASendTo API by calling
syscall.Syscall9 directly. This allows us to comply with

(4) Conversion of a Pointer to a uintptr when calling syscall.Syscall

rule.

After this change, this commands succeeds:

go test -a -short -gcflags=all=-d=checkptr -run=TestPacketConn net

Updates #34972

Change-Id: Ib9a810bedf9e05251b7d3c7f69e15bfbd177ac62
Reviewed-on: https://go-review.googlesource.com/c/go/+/220544


Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>