I wrote this patch to address two problems:

  1. if I do `mach run` from a directory other than $topsrcdir, $objdir, or
     $objdir/dist/bin, then .gdbinit will not be loaded.
  2. Debugging the firefox binary will never load the JS prettyprinters in any
     case.

I believe this patch fixes other problems as well, such as .gdbinit_python not being found, and the gdbpp pretty-printers not getting loaded in various situations.

The main changes of this patch are:

  1. Move .gdbinit into build/ (and $objdir/build/) to delay it from getting
     loaded until the search path is configured.
  2. Move libxul.so-gdb.py into the correct directory.
  3. Use either libxul.so-gdb.py or js-gdb.py to configure the correct
     search path then load .gdbinit, and have .gdbinit load all of the
     pretty-printers (Gecko and JS).
  4. Use a single preprocessed file to configure the source directory. Use
     relative paths within the objdir for everything else.

Differential Revision: https://phabricator.services.mozilla.com/D77589