gdb+gdbserver/Linux: avoid reading registers while going through shell

For every stop, Linux GDB and GDBserver save the stopped thread's PC,
in lwp->stop_pc.  This is done in save_stop_reason, in both
gdb/linux-nat.c and gdbserver/linux-low.cc.  However, while we're
going through the shell after "run", in startup_inferior, we shouldn't
be reading registers, as we haven't yet determined the target's
architecture -- the shell's architecture may not even be the same as
the final inferior's.

In gdb/linux-nat.c, lwp->stop_pc is only needed when the thread has
stopped for a breakpoint, and since when going through the shell, no
breakpoint is going to hit, we could simply teach save_stop_reason to
only record the stop pc when the thread stopped for a breakpoint.

However, in gdbserver/linux-low.cc, lwp->stop_pc is used in more cases
than breakpoint hits (e.g., it's used in tracepoints & the
"while-stepping" feature).

So to avoid GDB vs GDBserver divergence, we apply the same approach to
both implementations.

We set a flag in the inferior (process in GDBserver) whenever it is
being nursed through the shell, and when that flag is set,
save_stop_reason bails out early.  While going through the shell,
we'll only ever get process exits (normal or signalled), random
signals, and exec events, so nothing is lost.

Change-Id: If0f01831514d3a74d17efd102875de7d2c6401ad

gdb/fork-child.c

@@ -126,6 +126,9 @@ gdb_startup_inferior (pid_t pid, int num_traps)
   inferior *inf = current_inferior ();
   process_stratum_target *proc_target = inf->process_target ();
 
+  scoped_restore save_starting_up
+    = make_scoped_restore (&inf->starting_up, true);
+
   ptid_t ptid = startup_inferior (proc_target, pid, num_traps, NULL, NULL);
 
   /* Mark all threads non-executing.  */