Fix gdb.gdb/ selftest tests when testing optimized GDB builds

After commit bf4692711232 ("Eliminate catch_errors"), GCC started
inlining captured_command_loop in captured_main.  And setting a
breakpoint on captured_command_loop makes the inferior GDB stop in
captured_main, _after_ captured_command_loop's call to
interp_pre_command_loop, which prints the inferior GDB's prompt, has
already executed, confusing the gdb.gdb/ selftest tests:

  (gdb) FAIL: gdb.gdb/complaints.exp: run until breakpoint at captured_command_loop
  WARNING: Couldn't test self

Debugging GDB with GDB manually, we see:

  (top-gdb) b captured_command_loop
  Breakpoint 1 at 0x71ee60: file src/gdb/main.c, line 324.
  (top-gdb) r
  [....]
  (gdb)                  <<<<<< PROMPT HERE
  Thread 1 "gdb" hit Breakpoint 1, captured_main (data=<optimized out>) at src/gdb/main.c:1147
  1147              captured_command_loop ();
  (top-gdb)

Note the stop at 'captured_main', and the "PROMPT HERE" line.  That
prompt does not show up when debugging a non-optimized build of GDB.

Fix this by preventing inlining of captured_command_loop.

Ref: https://sourceware.org/ml/gdb-patches/2017-10/msg00522.html

gdb/ChangeLog:
2017-10-20  Pedro Alves  <palves@redhat.com>

	* main.c (captured_command_loop): Add attribute noinline.

gdb/main.c

@@ -307,6 +307,11 @@ setup_alternate_signal_stack (void)
 
 /* Call command_loop.  */
 
+/* Prevent inlining this function for the benefit of GDB's selftests
+   in the testsuite.  Those tests want to run GDB under GDB and stop
+   here.  */
+static void captured_command_loop () __attribute__((noinline));
+
 static void
 captured_command_loop ()
 {