GDB (s12z): Improve reliability of the stack unwinder.

GDB (s12z): Improve reliability of the stack unwinder.

Previously, the stack unwinder searched through consecutive bytes for values
which it thought might be the start of a stack mutating operation.
This was error prone, because such bytes could also be the operands of other
instructions.  This change uses the opcodes api to interpret the code in each
frame.

gdb/ChangeLog:
	* s12z-tdep.c (push_pull_get_stack_adjustment): New function.
	(advance, posn, abstract_read_memory): New functions.
	[struct mem_read_abstraction]: New struct.
	(s12z_frame_cache): Use opcodes API to interpret stack frame code.