[release-branch.go1.12] cmd/link: macos: set initial protection of 0 for __DWARF segment

For later versions of MacOS, the dynamic loader is more picky about
enforcing restrictions on __DWARF MachO load commands/segments,
triggering aborts of the form

  dyld: malformed mach-o image: segment __DWARF has vmsize < filesize

for Go programs that use cgo on Darwin. The error is being triggered
because the Go linker is setting "vmsize" in the DWARF segment entry
to zero as a way to signal that the DWARF doesn't need to be mapped
into memory at runtime (which we need to continue to do).

This patch changes the initial protection on the __DWARF segment to
zero, which dyld seems to be happy with (this is used for other similar
non-loadable sections such as __LLVM).

Updates #32697

Change-Id: I9a73449c6d26c172f3d70361719943af381f37e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/182958
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/183397

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

@@ -429,6 +429,7 @@ func machoUpdateDwarfHeader(r *loadCmdReader, buildmode BuildMode, compressedSec
 	segv := reflect.ValueOf(seg).Elem()
 	segv.FieldByName("Offset").SetUint(uint64(dwarfstart))
 	segv.FieldByName("Addr").SetUint(uint64(dwarfaddr))
+	segv.FieldByName("Prot").SetUint(0)
 
 	if compressedSects != nil {
 		var segSize uint64