gas obj-coff memory leaks

gas obj-coff memory leaks

This patch addresses memory leaks in gas that show up when running the
testsuite on x86_64-w64-mingw32.  The seh_ctx_cur, and weak sym naming
leaks can occur many times during assembly.  The symbol hook and
section leaks are not so important since this memory needs to persist
until closing the output bfd.

	* config/obj-coff-seh.c (do_seh_endproc): Free seh_ctx_cur and
	its fields.
	* config/obj-coff-seh.h (struct seh_context): Remove unused
	"next" field.
	* config/obj-coff.c (coff_obj_symbol_new_hook): Use notes_alloc
	for aux entries.
	(coff_obj_symbol_clone_hook): Likewise.
	(obj_coff_def): Don't strdup name unless we need to do so
	for tc_canonicalize_symbol_name.  Free after making symbol.
	(weak_name2altname, weak_altname2name): Return a char*.
	(weak_uniquify): Use notes_concat.
	(pecoff_obj_set_weak_hook, pecoff_obj_clear_weak_hook): Free name
	returned by weak_name2altname.
	(coff_frob_symbol): Similarly for weak_altname2name.
	(obj_coff_section): Use notes_memdup0.
	* symbols.h: Add include guard.
	(notes_memdup0): New inline function.