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).

cgo/testdata/basic.out.go

@@ -1,5 +1,6 @@
 package main
 
+import "syscall"
 import "unsafe"
 
 var _ unsafe.Pointer
@@ -31,6 +32,13 @@ func C.CBytes(b []byte) unsafe.Pointer {
 	return C.__CBytes(b)
 }
 
+//go:linkname C.__get_errno_num runtime.cgo_errno
+func C.__get_errno_num() uintptr
+
+func C.__get_errno() error {
+	return syscall.Errno(C.__get_errno_num())
+}
+
 type (
 	C.char      uint8
 	C.schar     int8