[Bug boehm-gc/66848] boehm-gc fails test suite on x86_64-apple-darwin15

["howarth.at.gcc at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66848

--- Comment #15 from Jack Howarth <howarth.at.gcc at gmail dot com> ---
(In reply to Jack Howarth from comment #14)
> I finally got around to rebuilding the Apple open source release of
> libunwind-35.3 from 10.10.5 under Xcode 7 on 10.10.5. The results are rather
> interesting as the default build is a Debug one compiled at -O0. The debug
> build of libunwind.dylib produces a binary which exhibits the same breakage
> in the boehm-gc test suite binaries built on darwin14 as is seen on darwin15
> with the optimized system libunwind.dylib. This makes it much more likely
> that the issue isn't an optimization bug in Apple Clang 7.0 but rather a
> linker bug in Xcode 7. Unfortunately, it is impossible to test that by
> linking the Xcode 7 build under the Xcode 6 linker because the Xcode 7 build
> uses the 10.11 SDK on 10.10 which needs a linkage on libc++abi.tbd and thus
> requires the new linker with the .tbd support.
> 
> FYI, The .tbd files are new "text-based stub libraries", that provide a much
> more compact version of the stub libraries for use in the SDK, and help to
> significantly reduce its download size.

Nick,
     Are there any changes in the default behavior of the linker from Xcode 6.4
to 7.0 which I can revert in my Xcode 7 build of libunwind-35.3 on 10.10.5 with
linker flags?
          Jack