…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

…tection fix

Tweak the previous fix for issue 32673 (in CL 182958) to work around
problems with c-shared build mode that crop up on some of the builders
(10.11, 10.12).  We now consistently set vmaddr and vmsize to zero
for the DWARF segment regardless of build mode.

Fixes #32697

Change-Id: Id1fc213590ad00c28352925e2d754d760e022b5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/183237
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/183398
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>