Use libiberty hash in gas/read.c.

Use libiberty hash in gas/read.c.

	* read.c (struct po_entry): New.
	(hash_po_entry): Likewise.
	(eq_po_entry): Likewise.
	(po_entry_alloc): Likewise.
	(po_entry_find): Likewise.
	(pop_insert): Likewise.
	(pobegin): Use htab hash table.
	(read_a_source_file): Likewise.
	(s_macro): Likewise.
	(read_print_statistics): Likewise.
	* config/tc-m68k.c (m68k_conditional_pseudoop): Add const qualifier.
	* config/tc-m68k.h (m68k_conditional_pseudoop): Likewise.