runtime,syscall: move SyscallX implementations from runtime to syscall

runtime,syscall: move SyscallX implementations from runtime to syscall

There is no need for syscall.Syscall{3,6,9,12,15,18} to be implemented
in the runtime and linknamed from syscall. All of them can be
implemented using the single runtime.syscall_syscalln function.

While here, improve the documentation of syscall.SyscallN.

Change-Id: I0e09d42e855d6baf900354c9b7992a4329c4ffc7
Reviewed-on: https://go-review.googlesource.com/c/go/+/690515
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>