[gdb/breakpoints] Handle glibc with debuginfo in create_exception_master_breakpoint

The test-case nextoverthrow.exp is failing on targets with unstripped libc.

This is a regression since commit 1940319c "[gdb] Fix internal-error in
process_event_stop_test".

The problem is that this code in create_exception_master_breakpoint:
...
      for (objfile *sepdebug = obj->separate_debug_objfile;
      	   sepdebug != nullptr; sepdebug = sepdebug->separate_debug_objfile)
        if (create_exception_master_breakpoint_hook (sepdebug))
...
iterates over all the separate debug object files, but fails to handle the
case that obj itself has the debug info we're looking for.

Fix this by using the separate_debug_objfiles () range instead, which does
iterate both over obj and the obj->separate_debug_objfile chain.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-02-05  Tom de Vries  <tdevries@suse.de>

	PR breakpoints/27330
	* breakpoint.c (create_exception_master_breakpoint): Handle case that
	glibc object file has debug info.