Convert target_structs vector to VEC

Convert target_structs vector to VEC

I thought that this home made implementation of a vector could be
replaced by the more standard VEC. The implementation seems to predate
the introduction of vec.h, so that would explain why it exists.

Ran make check before and after, no new failures.

gdb/ChangeLog:

2014-08-19  Simon Marchi  <simon.marchi@ericsson.com>

	* target.c (target_struct_size): Remove.
	(target_struct_allocsize): Remove.
	(DEFAULT_ALLOCSIZE): Remove.
	(target_ops_p): New typedef.
	(DEF_VEC_P (target_ops_p)): New vector type.
	(target_structs): Change type to VEC (target_ops_p).
	(add_target_with_completer): Replace "push" code by VEC_safe_push.
	(find_default_run_target): Rewrite for loop following changes to
	target_structs.