bug#46256: [feature/native-comp] AOT eln files ignored if run from build tree

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 03 Mar 2021 20:27:55 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 46256@debbugs.gnu.org
> 
>  . After fixing the above, Emacs starts, but as soon as some simple
>    command is invoked, and Emacs starts native-compiling Lisp
>    packages, the Emacs subprocesses which run the async compilation
>    start crashing.  Not all of them crash, but some do.  I wasn't yet
>    able to find where they crash or why; stay tuned.

Some more info about these crashes: I see this in the *Messages*
buffer when a compilation crashes:

       Warning (comp): comp.h:70: Emacs fatal error: assertion failed: NATIVE_COMP_UNITP (a)^M

I also see similar picture in emacs_backtrace.txt:

  emacs_abort at src/w32fns.c:10947
  terminate_due_to_signal at src/emacs.c:417
  die at src/alloc.c:7452
  XNATIVE_COMP_UNIT at src/comp.h:70
  load_comp_unit at src/comp.c:4766
  syms_of_comp at src/comp.c:5077
  Fload at src/lread.c:1548

(My Emacs is compiled with --enable-checking=yes.)

Btw, that ^M character after the error message probably means we don't
correctly decode messages from the async compilation subprocesses --
but this is a secondary problem for now.