breakpoint shadowing, take single-step breakpoints into account.

Breakpoints are supposed to be transparent to memory accesses.  For
all kinds of breakpoints breakpoint_xfer_memory hides the breakpoint
instructions.  However, sss breakpoints aren't tracked like all other
breakpoints, and nothing is taking care of hiding them from memory
reads.

Say, as is, a background step + disassemble will see breakpoints
instructions on software step targets.  E.g., stepping over this line:

  while (1);

with s&

and then "disassemble" would show sss breakpoints.

Actually, that's still not be possible to see today, because:

 - in native Linux, you can't read memory while the program
   is running.
 - with Linux gdbserver, you can, but in the all-stop RSP you
   can't talk to the server while the program is running...
 - and with non-stop, on software step targets, we presently
   force the use of displaced-stepping for all single-steps,
   so no single-step breakpoints are used...

I've been working towards making non-stop not force displaced stepping
on sss targets, and I noticed the issue then.  With that, I indeed see
this:

(gdb) set remote Z-packet off
(gdb) s&
(gdb) disassemble main
Dump of assembler code for function main:
   0x000000000040049c <+0>:     push   %rbp
   0x000000000040049d <+1>:     mov    %rsp,%rbp
   0x00000000004004a0 <+4>:     int3
   0x00000000004004a1 <+5>:     (bad)
End of assembler dump.

Instead of the correct:

(gdb) disassemble main
Dump of assembler code for function main:
   0x000000000040049c <+0>:     push   %rbp
   0x000000000040049d <+1>:     mov    %rsp,%rbp
   0x00000000004004a0 <+4>:     jmp    0x4004a0 <main+4>

This is actually one thing that my v1 of the recent "fix a bunch of
run control bugs" series was fixing, because it made sss breakpoints
be regular breakpoints in the breakpoint chain.  But dropped it in the
version that landed in the tree, due to some problems.

So instead of making sss breakpoints regular breakpoints, go with a
simpler fix (at least for now) -- make breakpoint_xfer_memory take
software single-step breakpoints into account.  After the patch, I get
the correct disassemble output.

Tested on x86_64 Fedora 17, and also on top of my "use software
single-step on x86" series.

Also fixes the issue pointed out by Yao at
https://sourceware.org/ml/gdb-patches/2014-04/msg00045.html, where the
prologue analysis/frame sniffing manages to see software step
breakpoint instructions.

gdb/
2014-04-10  Pedro Alves  <palves@redhat.com>

	* breakpoint.c (single_step_breakpoints)
	(single_step_gdbarch): Move up in the file.
	(one_breakpoint_xfer_memory): New function, factored out from ...
	(breakpoint_xfer_memory): ... here.  Also process single-step
	breakpoints.