cmd/link: only adjust pagezero for iOS on darwin/arm64

The new pagezero_size introduced by CL 72730 breaks
on 32-bit systems, since it is 2³². Restrict the change to
darwin/arm64, since it is intended for iOS only.

We could plausibly allow GOARCH=amd64 as well, but
without a compelling reason, changing the zero page size
doesn't seem worth the risk.

Change-Id: I5d6adcbaff8d0e5b169ff13512f188332cc7ed9a
Reviewed-on: https://go-review.googlesource.com/73250
Run-TryBot: Russ Cox <rsc@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/link/internal/ld/lib.go

@@ -1105,10 +1105,13 @@ func (ctxt *Link) hostlink() {
 	switch ctxt.BuildMode {
 	case BuildModeExe:
 		if ctxt.HeadType == objabi.Hdarwin {
-			// __PAGEZERO segment size determined empirically.
-			// XCode 9.0.1 successfully uploads an iOS app with this value.
-			// Also works for macOS apps.
-			argv = append(argv, "-Wl,-pagezero_size,100000000")
+			if ctxt.Arch.Family == sys.ARM64 {
+				// __PAGEZERO segment size determined empirically.
+				// XCode 9.0.1 successfully uploads an iOS app with this value.
+				argv = append(argv, "-Wl,-pagezero_size,100000000")
+			} else {
+				argv = append(argv, "-Wl,-pagezero_size,4000000")
+			}
 		}
 	case BuildModePIE:
 		// ELF.