ppc gas: don't leak ppc_hash memory

	* config/tc-ppc.c (insn_obstack): New.
	(insn_calloc): New function.
	(ppc_setup_opcodes): Use insn_obstack for ppc_hash.

(cherry picked from commit a887be69)
Without ppc_md_end since the infrastructure to call that late isn't
available on the branch, and without the multiply overflow check.