syscall: restore nosplit for ptrace1 on Darwin

ptrace1 must be nosplit because it is called from
forAndExecInChild. It was marked nosplit in the generated code
but not in the generator. CL 313230 regenerated the code and lost
the nosplit mark. This CL restores it.

Change-Id: I4645d83650f1818bed3cb650328bba97074b6b2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/314249


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>