Ship debug information in a separate file from the shared library

By shipping the debug information in a separate file, downstream
packages (e.g. the Ruby packages) can easily remove the debug
information while packaging libddprof, but it's still very easy to
restore it.

This approach is documented in
<https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html>
and in the `objdump` man pages. Thanks @nsavoire for the tips.

The `strip`'d file retains the `build-id` information.

I've tested with `gdb` and it seems to work:

```
$ gdb libddprof_ffi.so
GNU gdb (Ubuntu 11.1-0ubuntu2) 11.1

Reading symbols from libddprof_ffi.so...
Reading symbols from /working/build-linux-amd64/lib/libddprof_ffi.so.debug...

(gdb) info functions
All defined functions:

File /cargo/registry/src/git.luolix.top-1ecc6299db9ec823/addr2line-0.13.0/src/lib.rs:
966:	static fn addr2line::Function::parse_children<gimli::read::endian_slice::EndianSlice<gimli::endianity::LittleEndian>>();
549:	static fn addr2line::ResUnit::parse_functions<gimli::read::endian_slice::EndianSlice<gimli::endianity::LittleEndian>>();
480:	static fn addr2line::ResUnit::parse_lines<gimli::read::endian_slice::EndianSlice<gimli::endianity::LittleEndian>>();
660:	static fn addr2line::name_attr<gimli::read::endian_slice::EndianSlice<gimli::endianity::LittleEndian>>();
1162:	static fn addr2line::{{impl}}::for_each_range::{{closure}}<gimli::read::endian_slice::EndianSlice<gimli::endianity::LittleEndian>,closure-0>();

File /cargo/registry/src/git.luolix.top-1ecc6299db9ec823/compiler_builtins-0.1.32/src/macros.rs:
262:	fn compiler_builtins::int::udiv::__udivmodti4();
269:	fn compiler_builtins::int::udiv::__umodti3::__umodti3();
...etc...
```

and if the `.debug` file is deleted:

```
(gdb) info functions
All defined functions:

Non-debugging symbols:
0x0000000000075d00  _init
0x0000000000075d30  __stack_chk_fail@plt
0x0000000000075d40  __tls_get_addr@plt
0x0000000000075d50  __fxstat64@plt
0x0000000000075d60  __gmon_start__@plt
0x0000000000075d70  _Unwind_Resume@plt
0x0000000000075d80  __cxa_finalize@plt
0x0000000000075dc0  deregister_tm_clones
0x0000000000075df0  register_tm_clones
0x0000000000075e30  __do_global_dtors_aux
0x0000000000075e70  frame_dummy
0x0000000000075ea5  std::error::<impl core::convert::From<E> for alloc::boxed::Box<dyn std::error::Error>>::from
0x0000000000075edd  std::error::Error::description
0x0000000000075eea  std::error::Error::cause
...etc...
```

This PR is an alternative to #28, which instead shipped two copies
of the shared library binaries (one `strip`d, and an untouched one).

ffi-build.sh

@@ -84,6 +84,12 @@ fi
 if command -v objcopy > /dev/null; then
     # Remove .llvmbc section which is not useful for clients
     objcopy --remove-section .llvmbc "$destdir/lib/libddprof_ffi.a"
+
+    # Ship debug information separate from shared library, so that downstream packages can selectively include it
+    # https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html
+    objcopy --only-keep-debug "$destdir/lib/libddprof_ffi${shared_library_suffix}" "$destdir/lib/libddprof_ffi${shared_library_suffix}.debug"
+    strip -S "$destdir/lib/libddprof_ffi${shared_library_suffix}"
+    objcopy --add-gnu-debuglink="$destdir/lib/libddprof_ffi${shared_library_suffix}.debug" "$destdir/lib/libddprof_ffi${shared_library_suffix}"
 fi
 
 echo "Checking that native-static-libs are as expected for this platform..."