cmd/go: compile runtime/internal/syscall as a runtime package

cmd/go: compile runtime/internal/syscall as a runtime package

runtime/internal/syscall is a runtime package, so it should be built
with -+.

Specifically, we don't want libfuzzer instrumentation in Go functions
defined in runtime/internal/syscall, which is disabled with -+.

For #53190.

Change-Id: I9f16f5c7c7ce10b98371e9de82fcea6da854e163
Reviewed-on: https://go-review.googlesource.com/c/go/+/413818
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>