[gdb/cli] Add maint info frame-unwinders

[gdb/cli] Add maint info frame-unwinders

Add a new command "maint info frame-unwinders":
...
(gdb) help maint info frame-unwinders
List the frame unwinders currently in effect, starting with the highest \
  priority.
...

Output for i386:
...
$ gdb -q -batch -ex "set arch i386" -ex "maint info frame-unwinders"
The target architecture is set to "i386".
dummy                   DUMMY_FRAME
dwarf2 tailcall         TAILCALL_FRAME
inline                  INLINE_FRAME
i386 epilogue           NORMAL_FRAME
dwarf2                  NORMAL_FRAME
dwarf2 signal           SIGTRAMP_FRAME
i386 stack tramp        NORMAL_FRAME
i386 sigtramp           SIGTRAMP_FRAME
i386 prologue           NORMAL_FRAME
...

Output for x86_64:
...
$ gdb -q -batch -ex "set arch i386:x86-64" -ex "maint info frame-unwinders"
The target architecture is set to "i386:x86-64".
dummy                   DUMMY_FRAME
dwarf2 tailcall         TAILCALL_FRAME
inline                  INLINE_FRAME
python                  NORMAL_FRAME
amd64 epilogue          NORMAL_FRAME
i386 epilogue           NORMAL_FRAME
dwarf2                  NORMAL_FRAME
dwarf2 signal           SIGTRAMP_FRAME
amd64 sigtramp          SIGTRAMP_FRAME
amd64 prologue          NORMAL_FRAME
i386 stack tramp        NORMAL_FRAME
i386 sigtramp           SIGTRAMP_FRAME
i386 prologue           NORMAL_FRAME
...

Tested on x86_64-linux.

Reviewed-By: Tom Tromey <tom@tromey.com>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>