runtime/cgo: fix clang mach_port_t cast warning

runtime/cgo: fix clang mach_port_t cast warning

Converting *void directly to mach_port_t causes newer clang to throw a
void-pointer-to-int-cast warning/error.

Change-Id: I709955d4678bed3f690a8337ce85fd8678d217bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/573415
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>