cgo: support errno value as second return parameter

Making this work on all targets was interesting but there's now a test
in place to make sure this works on all targets that have the CGo test
enabled (which is almost all targets).

src/runtime/os_windows.go

@@ -113,3 +113,6 @@ func syscall_Getpagesize() int {
 	_GetSystemInfo(unsafe.Pointer(&info))
 	return int(info.dwpagesize)
 }
+
+//export _errno
+func libc_errno_location() *int32