libdw: dwarf_frame_register takes an array of at least 3 Dwarf_Ops

libdw: dwarf_frame_register takes an array of at least 3 Dwarf_Ops

GCC11 will warn about a mismatch in the declaration of dwarf_frame_register:

dwarf_frame_register.c:37:61: error: argument 3 of type ‘Dwarf_Op *’
  declared as a pointer [-Werror=array-parameter=]
   37 | dwarf_frame_register (Dwarf_Frame *fs, int regno, Dwarf_Op *ops_mem,
      |                                                   ~~~~~~~~~~^~~~~~~
libdw.h:1068:43: note: previously declared as an array ‘Dwarf_Op[3]’
 1068 |                                  Dwarf_Op ops_mem[3],
      |                                  ~~~~~~~~~^~~~~~~~~~

When fixing that it will show an actual bug in the addrcfi testcase:

addrcfi.c:98:16: error: ‘dwarf_frame_register’ accessing 96 bytes in a
  region of size 64 [-Werror=stringop-overflow=]
   98 |   int result = dwarf_frame_register (stuff->frame, regno, ops_mem, &ops, &nops);
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
addrcfi.c:98:16: note: referencing argument 3 of type ‘Dwarf_Op *’
 1069 | extern int dwarf_frame_register (Dwarf_Frame *frame, int regno,
      |            ^~~~~~~~~~~~~~~~~~~~

Fix the declaration, fix the bug and add an extra comment to the description
in libdw.h.

Signed-off-by: Mark Wielaard <mark@klomp.org>