gccNGPackages.libbacktrace: fix tests on darwin

The libbacktrace test suite was failing on darwin because the
btest_gnudebuglink tests attempted to use GNU debuglink functionality,
which only works on ELF binaries, not Mach-O on darwin.

We set `libbacktrace_cv_objcopy_debuglink=no`, allowing the remaining 21 tests to run successfully.

pkgs/development/compilers/gcc/ng/common/libbacktrace/default.nix

@@ -47,6 +47,11 @@ stdenv.mkDerivation (finalAttrs: {
     mkdir ../../build
     cd ../../build
     configureScript=../$sourceRoot/configure
+  ''
+  + lib.optionalString stdenv.hostPlatform.isDarwin ''
+    # GNU debuglink is not supported on macOS (Mach-O format)
+    # Skip the gnudebuglink tests which fail on Darwin
+    export libbacktrace_cv_objcopy_debuglink=no
   '';
 
   installPhase = ''