gdb: disable werror

-Werror should not be used when packaging.

Fixes build for some cross-compilation scenarios,
such as darwin -> i686-linux

pkgs/development/tools/misc/gdb/default.nix

···
     # subset of the platform description.
     "--program-prefix=${targetPrefix}"
 
+    "--disable-werror"
     "--enable-targets=all" "--enable-64-bit-bfd"
     "--disable-install-libbfd"
     "--disable-shared" "--enable-static"