Remove a VEC from remote.c

Remove a VEC from remote.c

This removes the VEC from remote_g_packet_data, replacing it with a
std::vector.  This is a bit odd in that this object is never
destroyed, and is obstack-allocated.  I believe a gdbarch is never
destroyed, so this seemed ok.

Tested by the buildbot.

gdb/ChangeLog
2018-11-09  Tom Tromey  <tom@tromey.com>

	* remote.c (remote_g_packet_guess_s): Remove typedef and DEF_VEC.
	(struct remote_g_packet_data): Derive from allocate_on_obstack.
	<guesses>: Now a std::vector.
	(remote_g_packet_data_init, register_remote_g_packet_guess):
	Update.
	(remote_read_description_p): Update.  Return bool.
	(remote_target::read_description): Update.
	(struct remote_g_packet_guess): Add constructor.