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).