cmd/link/internal/ld: set MachO platform to PLATFORM_IOS on iOS

CL 205340 changed the linker to skip certain flags when linking
for iOS. However, for host linking on iOS (such as on the Corellium
darwin/arm64 builders) the MachO platform defaults to PLATFORM_MACOS,
and the flags are not skipped.

Avoids warnings such as

    ld: warning: -no_pie ignored for arm64

Updates #32963

Change-Id: Ib6b4c2375fd14cf89410bf5ff1537b692b7a1c15
Reviewed-on: https://go-review.googlesource.com/c/go/+/206337
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -404,21 +404,26 @@ func (ctxt *Link) domacho() {
 		}
 	}
 	if machoPlatform == 0 {
-		machoPlatform = PLATFORM_MACOS
-		if ctxt.LinkMode == LinkInternal {
-			// For lldb, must say LC_VERSION_MIN_MACOSX or else
-			// it won't know that this Mach-O binary is from OS X
-			// (could be iOS or WatchOS instead).
-			// Go on iOS uses linkmode=external, and linkmode=external
-			// adds this itself. So we only need this code for linkmode=internal
-			// and we can assume OS X.
-			//
-			// See golang.org/issues/12941.
-			//
-			// The version must be at least 10.9; see golang.org/issues/30488.
-			ml := newMachoLoad(ctxt.Arch, LC_VERSION_MIN_MACOSX, 2)
-			ml.data[0] = 10<<16 | 9<<8 | 0<<0 // OS X version 10.9.0
-			ml.data[1] = 10<<16 | 9<<8 | 0<<0 // SDK 10.9.0
+		switch ctxt.Arch.Family {
+		default:
+			machoPlatform = PLATFORM_MACOS
+			if ctxt.LinkMode == LinkInternal {
+				// For lldb, must say LC_VERSION_MIN_MACOSX or else
+				// it won't know that this Mach-O binary is from OS X
+				// (could be iOS or WatchOS instead).
+				// Go on iOS uses linkmode=external, and linkmode=external
+				// adds this itself. So we only need this code for linkmode=internal
+				// and we can assume OS X.
+				//
+				// See golang.org/issues/12941.
+				//
+				// The version must be at least 10.9; see golang.org/issues/30488.
+				ml := newMachoLoad(ctxt.Arch, LC_VERSION_MIN_MACOSX, 2)
+				ml.data[0] = 10<<16 | 9<<8 | 0<<0 // OS X version 10.9.0
+				ml.data[1] = 10<<16 | 9<<8 | 0<<0 // SDK 10.9.0
+			}
+		case sys.ARM, sys.ARM64:
+			machoPlatform = PLATFORM_IOS
 		}
 	}