syscall: fix wrong unsafe.Pointer alignment in syscall

syscall: fix wrong unsafe.Pointer alignment in syscall

Caught with:

	go test -a -short -gcflags=all=-d=checkptr log/syslog

and:

	grep -rE '\*\[([^2]|.{2,})\].*\)\(unsafe.Pointer' syscall

Updates #34972

Change-Id: Iafd199b3a34beb7cc3e88484bf2fbae45183f951
Reviewed-on: https://go-review.googlesource.com/c/go/+/201877
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>