runtime: avoid receiving preemotion signal while exec'ing

The iOS kernel has the same problem as the macOS kernel. Extend
the workaround of #41702 (CL 262438 and CL 262817) to iOS.

Updates #35851.

Change-Id: I7ccec00dc96643c08c5be8b385394856d0fa0f64
Reviewed-on: https://go-review.googlesource.com/c/go/+/275293


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>