GDB w/ Mac OS cores: "no core file handler recognizes format"

Hi all.  As you probably know the latest Xcode does not provide GDB
anymore; they've switched to LLDB.  We've had nothing but head-banging
frustration with LLDB and have given up on it.  So I tried compiling the
latest GDB from source.  I understand the issues with signing the binary
so I can use it with a running program, but first I just want to be able
to read a coredump.

Unfortunately I can't get it to work at all; I always get this error:

  $ ./x86_64-darwin/bin/gdb -c core.54937 myprog
  GNU gdb (GDB) 7.6.2
   ...
  Reading symbols from myprog...Reading symbols from
myprog.dSYM/Contents/Resources/DWARF/myprog...done.
  done.
  "core.54937": no core file handler recognizes format

I've tried cores created by programs compiled with older versions of
clang that still used GCC as the backend, as well as those compiled with
newer, LLVM-based clang.

I've also tried installing the GDB's provided by HomeBrew etc. and I get
the same error with those.

I've googled all around looking for this error message and seen it a few
times but in totally unrelated contexts.  I can't believe that this
would go completely unreported if it was a common problem so it seems
like something unique about my setup, but I've no idea what's wrong.

Can anyone suggest any next steps for me to take?