> In GNU Emacs 27.0.90 (build 1, i686-apple-darwin10.0.0, NS appkit-1561.61 Version 10.13.6 (Build 17G11023))

"i686"? Are you building a 32-bit executable? If so, does the problem go away if 
you build the more-typical 64-bit executable?

What compiler and version are you using? And what GDB version are you using?

> (gdb) p args[0]
> $25 = XIL(0x3960)
> (gdb) xtype
> Lisp_Symbol
> (gdb) xsymbol
> $26 = (struct Lisp_Symbol *) 0x3af8
> Cannot access memory at address 0x3b00

Something's wrong here. xsymbol should have output something like:

$8 = (struct Lisp_Symbol *) 0x10515f0 <lispsym+14688>

Try this instead (my examples show output on my Ubuntu 18.04 x86-64 platform):

(gdb) p args[0]
$25 = XIL(0x3960)
(gdb) xgetsym $25
(gdb) p $ptr
$27 = (struct Lisp_Symbol *) 0x10515f0 <lispsym+14688>

I assume yours will output something like "$27 = (struct Lisp_Symbol *) 0x3af8" 
instead. If so, try this:

(gdb) xgetptr $25
(gdb) p/x $ptr
$28 = 0x3960

Assuming you get that, try this:

(gdb) p ((struct Lisp_Symbol *) ((char *)lispsym + 0x3960))
$29 = (struct Lisp_Symbol *) 0x10515f0 <lispsym+14688>

I assume yours will output something like "$29 = (struct Lisp_Symbol *) 0x3af8" 
instead. If so, please try this:

(gdb) p (char *)lispsym
$30 = 0x104dc90 <lispsym> "\230\001"
(gdb) p (char *)&lispsym[0]
$31 = 0x104dc90 <lispsym> "\230\001"

My *guess* is that the above two lines will differ due either to a bug in your 
compiler or in your debugger. Your $30 will equal 408. If I'm right, the 
attached patch should work around your compiler/debugger bug. Of course this 
will merely help you debug Emacs; it won't fix the original bug you reported. So 
if this fixes your problem please redo the backtrace info Eli asked for.