cmd/compile: add "runtime/internal/syscall" back to runtimePackages

cmd/compile: add "runtime/internal/syscall" back to runtimePackages

It was accidentally removed in CL 355451.

Change-Id: I818ea01b83e437c25829bf7e88c7180963e696f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/438615
Auto-Submit: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>