disassemble_free_powerpc

This fixes leaks in a ppc disassembler buffer.  I'm not sure now why I
used a private buffer for section contents, but I'm not going to
change that just now.

	* disassemble.h (disassemble_free_powerpc): Declare.
	* disassemble.c (disassemble_free_target): Call it.
	* ppc-dis.c (disassemble_free_powerpc): New function.