PR23938, should not free memory alloced in obstack by free()

PR23938, should not free memory alloced in obstack by free()

This removes ineffectual and wrong code caching section names in
gas/stabs.c.  Code like

  seg = subseg_new (name, 0);
  ...
  if (seg->name == name)
    seg->name = xstrdup (name);

with the idea of being able to unconditionally free "name" later no
longer works.  "name" is referenced by the section hash table as well
as in the section->name field.  It would be possible to use
"bfd_rename_section (stdoutput, seg, xstrdup (name))", but instead I
opted for a fairly straight-forward approach of adding extra
parameters to two functions to indicate section name strings should be
freed if possible.

	PR 23938
	* read.h (get_stab_string_offset): Update prototype.
	* stabs.c (get_stab_string_offset): Add free_stabstr_secname
	parameter.  Free stabstr_secname if unused as section name.
	Don't xstrdup name when used.
	(s_stab_generic): Remove forward declaration.  Add
	stab_secname_obstack_end param.  Reference notes obstack via
	macros.  Delete cached_secname.  Adjust get_stab_string_offset
	call.  Free stab_secname if unused as section name.
	(s_stab): Adjust s_stab_generic call.
	(s_xstab): Likewise.  Delete saved_secname and saved_strsecname.
	* config/obj-elf.c (obj_elf_init_stab_section): Adjust
	get_stab_string_offset call.
	* config/obj-coff.c (obj_coff_init_stab_section): Likewise.
	* config/obj-som.c (obj_som_init_stab_section): Likewise.
	* testsuite/gas/all/pr23938.s: New test.
	* testsuite/gas/all/gas.exp: Run it.