cmd/link/internal/ld: remove flags incompatible with -fembed-bitcode

The flags -headerpad, -Wl,-no_pie and -pagezero_size are incompatible with
the -fembed-bitcode flag used by `gomobile build`. Than McIntosh
suggested we might not need the offending flags; this change removes
the flags on darwin/arm64 and -headerpad, -pagezero_size on darwin/arm.

The -Wl,-no_pie flag is left for darwin/arm because linking fails
without it:

ld: warning: PIE disabled. Absolute addressing (perhaps -mdynamic-no-pic) not allowed in code signed PIE, but used in _runtime.rodata from /var/folders/qq/qxn86k813bn9fjxydm095rxw0000gp/T/workdir-host-darwin-amd64-zenly-ios/tmp/go-link-225285265/go.o. To fix this warning, don't compile with -mdynamic-no-pic or link with -Wl,-no_pie

Discussion: https://groups.google.com/d/msg/golang-dev/U1jK3xmmGAk/j0_ty46EDAAJ

I've verified the CL on the builders, built the "flappy" example from
gomobile with `gomobile build`, and verified that flappy runs on an
iPhone 5S.

Updates #32963

Change-Id: I783abc93ccf3c1d2b7ca00144b7164ba223d3529
Reviewed-on: https://go-review.googlesource.com/c/go/+/201358Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -1169,13 +1169,13 @@ func (ctxt *Link) hostlink() {
 
 	switch ctxt.HeadType {
 	case objabi.Hdarwin:
+		if !ctxt.Arch.InFamily(sys.ARM, sys.ARM64) {
+			// -headerpad is incompatible with -fembed-bitcode.
 			argv = append(argv, "-Wl,-headerpad,1144")
+		}
 		if ctxt.DynlinkingGo() && !ctxt.Arch.InFamily(sys.ARM, sys.ARM64) {
 			argv = append(argv, "-Wl,-flat_namespace")
 		}
-		if ctxt.BuildMode == BuildModeExe && !ctxt.Arch.InFamily(sys.ARM64) {
-			argv = append(argv, "-Wl,-no_pie")
-		}
 	case objabi.Hopenbsd:
 		argv = append(argv, "-Wl,-nopie")
 	case objabi.Hwindows:
@@ -1209,11 +1209,10 @@ func (ctxt *Link) hostlink() {
 	switch ctxt.BuildMode {
 	case BuildModeExe:
 		if ctxt.HeadType == objabi.Hdarwin {
-			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 {
+			if ctxt.Arch.Family != sys.ARM64 {
+				argv = append(argv, "-Wl,-no_pie")
+			}
+			if !ctxt.Arch.InFamily(sys.ARM, sys.ARM64) {
 				argv = append(argv, "-Wl,-pagezero_size,4000000")
 			}
 		}