i386 gas: don't leak op_hash or reg_hash memory

This tidies memory used by the two x86 gas string hash tables before
exiting.  I'm using a two-pronged approach, firstly the obvious call
to htab_delete plus telling the libiberty/hashtab.c infrastructure to
free tuples generated by str_hash_insert, and secondly putting the x86
core_optab memory on the notes obstack.  It would be possible to free
core_optab memory by using a custom hash table del_f on x86, as I do
for arc, but a later patch will move all the string hash memory to the
notes obstack.

	* config/tc-i386.c (md_begin): Use notes_alloc for core_optab.
	(386_md_end): New function.
	* config/tc-i386.h (386_md_end): Declare.
	(md_end): Define.
	* hash.h (str_htab_create): Pass free as del_f.