gdb: remove packet size limit

gdb: remove packet size limit

The remote packet buffer size is currently capped to 16384 mostly for
historical reasons, related to use of alloca.  Stop using alloca and
remove the limitation.

Tested on x86_64 Fedora 20.

gdb/ChangeLog:
2015-08-27  Pedro Alves  <palves@redhat.com>

	* remote.c (DEFAULT_MAX_MEMORY_PACKET_SIZE)
	(MIN_MEMORY_PACKET_SIZE): New.
	(MAX_REMOTE_PACKET_SIZE, MIN_REMOTE_PACKET_SIZE): Delete.
	(get_memory_packet_size): Adjust.  No longer limit the max packet
	size.
	(set_memory_packet_size): Adjust, and remove dead code.
	(remote_check_symbols): Use xmalloc and a cleanup instead of
	alloca.
	(remote_packet_size): No longer cap the packet size.
	(putpkt_binary): Use xmalloc and a cleanup instead of alloca.