Fix PR 21337: segfault when re-reading symbols.

Fix issue exposed by commit 3e29f34a.

The basic issue is that section data referenced through an objfile
pointer can also be referenced via the program-space data pointer,
although via a separate mapping mechanism, which is set up by
update_section_map.  Thus once section data attached to an objfile
pointer is released, the section map associated with the program-space
data pointer must be marked dirty to ensure that update_section_map is
called to prevent stale data being referenced.  For the matter at hand
this marking is being done via a call to objfiles_changed.

Before commit 3e29f34a objfiles_changed could be called after all of
the objfile pointers were processed in reread_symbols since section
data references via the program-space data pointer would not occur in
the calls of read_symbols performed by reread_symbols.

With commit 3e29f34a MIPS target specific calls to find_pc_section were
added to the code for DWARF information processing, which is called
via read_symbols.  Thus in reread_symbols the call to objfiles_changed
needs to be called before calling read_symbols, otherwise stale
section data can be referenced.

Thanks to Luis Machado for providing text for the main comment
associated with the change.

gdb/
2017-06-28  Doug Gilmore  <Doug.Gilmore@imgtec.com>
    PR gdb/21337
    * symfile.c (reread_symbols): Call objfiles_changed just before
    read_symbols.

gdb/testsuite/
2017-06-28  Doug Gilmore  <Doug.Gilmore@imgtec.com>
    PR gdb/21337
    * gdb.base/reread-readsym.exp: New file.
    * gdb.base/reread-readsym.c: New file.