syscall: allow for mksyscall_windows.go to be used outside of syscall
syscall: allow for mksyscall_windows.go to be used outside of syscall

Fixes issue 8398.

LGTM=chines
R=golang-codereviews, chines, josharian
CC=golang-codereviews
https://codereview.appspot.com/114140043

firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang