PR29256, memory leak in obj_elf_section_name

PR29256, memory leak in obj_elf_section_name

When handling section names in quotes obj_elf_section_name calls
demand_copy_C_string, which puts the name on the gas notes obstack.
Such strings aren't usually freed, since obstack_free frees all more
recently allocated objects as well as its arg.  When handling
non-quoted names, obj_elf_section_name mallocs the name.  Due to the
mix of allocation strategies it isn't possible for callers to free
names, if that was desirable.  Partially fix this by always creating
names on the obstack, which is more efficient anyway.  (You still
can't obstack_free on error paths due to the xtensa
tc_canonicalize_section_name.)  Also remove a couple of cases where
the name is dup'd for no good reason as far as I know.

	PR 29256
	* config/obj-elf.c (obj_elf_section_name): Create name on notes
	obstack.
	(obj_elf_attach_to_group): Don't strdup group name.
	(obj_elf_section): Likewise.
	(obj_elf_vendor_attribute): Use xmemdup0 rather than xstrndup.