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

@@ -404,6 +404,8 @@ func (ctxt *Link) domacho() {
 		}
 	}
 	if machoPlatform == 0 {
+		switch ctxt.Arch.Family {
+		default:
 			machoPlatform = PLATFORM_MACOS
 			if ctxt.LinkMode == LinkInternal {
 				// For lldb, must say LC_VERSION_MIN_MACOSX or else
@@ -420,6 +422,9 @@ func (ctxt *Link) domacho() {
 				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
+		}
 	}
 
 	// empirically, string table must begin with " \x00".