btrace: Count gaps as one instruction explicitly.

This gives all instructions, including gaps, a unique number.  Add a function
to retrieve the error code if a btrace instruction iterator points to an
invalid instruction.

Signed-off-by: Tim Wiederhake <tim.wiederhake@intel.com>

gdb/ChangeLog:

	* btrace.c (ftrace_call_num_insn, btrace_insn_get_error): New function.
	(ftrace_new_function, btrace_insn_number, btrace_insn_cmp,
	btrace_find_insn_by_number): Remove special case for gaps.
	* btrace.h (btrace_insn_get_error): New export.
	(btrace_insn_number, btrace_find_insn_by_number): Adjust comment.
	* record-btrace.c (btrace_insn_history): Print number for gaps.
	(record_btrace_info, record_btrace_goto): Handle gaps.

Change-Id: I8eb0e48a95f4278522fea74ea13526bfe6898ecc