[gdb] Fix internal-error in process_event_stop_test

The function create_exception_master_breakpoint in gdb/breakpoint.c attempts
to set a master exception breakpoint in each objfile.  It tries this using
a libgcc/unwind probe, and if that fails then using the
_Unwind_DebugHook symbol:
...
   for (objfile *objfile : current_program_space->objfiles ())
     {
        /* Try using probes.  */
        if (/* successful */)
          continue;

        /* Try using _Unwind_DebugHook */
     }
...

The preference scheme works ok both if the objfile has debug info, and if it's
stripped.

But it doesn't work when the objfile has a .gnu_debuglink to a .debug file
(and the .debug file is present).  What happens is that:
- we first encounter objfile libgcc.debug
- we try using probes, and this fails
- so we try _Unwind_DebugHook, which succeeds
- next we encounter objfile libgcc
- we try using probes, and this succeeds.
So, we end up with a master exception breakpoint in both libgcc (using probes)
and libgcc.debug (using _Unwind_DebugHook).

This eventually causes:
...
(gdb) PASS: gdb.cp/nextoverthrow.exp: post-check - next over a throw 3
next^M
src/gdb/infrun.c:6384: internal-error: \
  void process_event_stop_test(execution_control_state*): \
  Assertion `ecs->event_thread->control.exception_resume_breakpoint != NULL' \
  failed.^M
A problem internal to GDB has been detected,^M
further debugging may prove unreliable.^M
Quit this debugging session? (y or n) FAIL: gdb.cp/nextoverthrow.exp: next
past catch (GDB internal error)
...

To trigger this internal-error, we need to use gcc-10 or later to compile the
test-case, such that it contains the fix for gcc PR97774 - "Incorrect line
info for try/catch".

Fix this by only trying to install the master exception breakpoint in
libgcc.debug using the _Unwind_DebugHook method, if the install using probes
in libgcc failed.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-01-08  Tom de Vries  <tdevries@suse.de>

	PR gdb/26881
	* breakpoint.c (create_exception_master_breakpoint_probe)
	(create_exception_master_breakpoint_hook): Factor out
	of ...
	(create_exception_master_breakpoint): ... here.  Only try to install
	the master exception breakpoint in objfile.debug using the
	_Unwind_DebugHook method, if the install using probes in objfile
	failed.