The macOS and iOS external linker strips DWARF information from
binaries because it assumes the information will go into separate
DWARF information .dSYM files. To preserve the embedded debugging
information, the Go linker re-combines the separate DWARF
information into the unmapped __DWARF segment of the final
executable.

However, the iOS dyld linker does not allow unmapped segments, so
use the presence of the LC_VERSION_MIN_IPHONEOS linker command to
skip DWARF combining. Note that we can't use GOARCH for detection
since the iOS emulator runs on  GOARCH=386 and GOARCH=amd64 and we
will run into https://golang.org/issues/25148.

Updates #25148.

Change-Id: I29a1bc468fdee74ab3b27c46931501a0a8120c66
Reviewed-on: https://go-review.googlesource.com/111275
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>