btrace: fix gap indication

btrace: fix gap indication

Trace gaps due to overflows or non-contiguous trace are ignored in the 'info
record' command.  Fix that.

Also add a warning when decoding the trace and print the instruction number
preceding the trace gap in that warning message.  It looks like this:

    (gdb) info record
    Active record target: record-btrace
    Recording format: Intel Processor Trace.
    Buffer size: 16kB.
    warning: Decode error (-13) at instruction 101044 (offset = 0x29f0, pc = 0x7ffff728a642): no memory mapped at this address.
    Recorded 101044 instructions in 2093 functions (1 gaps) for thread 1 (process 5360).
    (gdb) record instruction-history 101044
    101044     0x00007ffff728a640:  pop    %r13
    [decode error (-13): no memory mapped at this address]

Remove the dead code that was supposed to print a gaps warning at the end of
trace decode.  This isn't really needed since we now print a warning for each
gap.

gdb/
	* btrace.c (ftrace_add_pt): Fix gap indication.  Add warning for non-
	contiguous trace and overflow.  Rephrase trace decode warning and print
	instruction number.  Remove dead gaps warning.
	(btrace_compute_ftrace_bts): Rephrase warnings and print instruction
	number.