Fix longjmp across readline w/ --enable-sjlj-exceptions toolchains

Nowadays, GDB propagates C++ exceptions across readline using
setjmp/longjmp 8952576 ("Propagate GDB/C++ exceptions across
readline using sj/lj-based TRY/CATCH") because DWARF-based unwinding
can't cross C functions compiled without -fexceptions (see details
from the commit above).

Unfortunately, toolchains that use SjLj-based C++ exceptions got
broken with that fix, because _Unwind_SjLj_Unregister, which is put at
the exit of a function, is not executed due to the longjmp added by
that commit.

 (gdb) [New Thread 2936.0xb80]
 kill

 Thread 1 received signal SIGSEGV, Segmentation fault.
 0x03ff662b in ?? ()
 top?bt 15
 #0  0x03ff662b in ?? ()
 #1  0x00526b92 in stdin_event_handler (error=0, client_data=0x172ed8)
    at ../../binutils-gdb/gdb/event-top.c:555
 #2  0x00525a94 in handle_file_event (ready_mask=<optimized out>,
    file_ptr=0x3ff5cb8) at ../../binutils-gdb/gdb/event-loop.c:733
 #3  gdb_wait_for_event (block=block@entry=1)
    at ../../binutils-gdb/gdb/event-loop.c:884
 #4  0x00525bfb in gdb_do_one_event ()
    at ../../binutils-gdb/gdb/event-loop.c:347
 #5  0x00525ce5 in start_event_loop ()
    at ../../binutils-gdb/gdb/event-loop.c:371
 #6  0x0051fada in captured_command_loop (data=0x0)
    at ../../binutils-gdb/gdb/main.c:324
 #7  0x0051cf5d in catch_errors (
    func=func@entry=0x51fab0 <captured_command_loop(void*)>,
    func_args=func_args@entry=0x0,
    errstring=errstring@entry=0x7922bf <VEC_interp_factory_p_quick_push(VEC_inte rp_factory_p*, interp_factory*, char const*, unsigned int)::__PRETTY_FUNCTION__+351> "", mask=mask@entry=RETURN_MASK_ALL)
    at ../../binutils-gdb/gdb/exceptions.c:236
 #8  0x00520f0c in captured_main (data=0x328feb4)
    at ../../binutils-gdb/gdb/main.c:1149
 #9  gdb_main (args=args@entry=0x328feb4) at ../../binutils-gdb/gdb/main.c:1159
 #10 0x0071e400 in main (argc=1, argv=0x171220)
    at ../../binutils-gdb/gdb/gdb.c:32

Fix this by making the functions involved in setjmp/longjmp as
noexcept, so that the compiler knows it doesn't need to emit the
_Unwind_SjLj_Register / _Unwind_SjLj_Unregister calls for C++
exceptions.

Tested on x86_64 Fedora 23 with:
 - GCC 5.3.1 w/ DWARF-based exceptions.
 - GCC 7 built with --enable-sjlj-exceptions.

gdb/ChangeLog:
2016-12-20  Pedro Alves  <palves@redhat.com>
	    Yao Qi  <yao.qi@linaro.org>

	PR gdb/20977
	* event-top.c (gdb_rl_callback_read_char_wrapper_noexcept): New
	noexcept function, factored out from ...
	(gdb_rl_callback_read_char_wrapper): ... this.
	(gdb_rl_callback_handler): Mark noexcept.

gdb/ChangeLog

@@ -1,3 +1,12 @@
+2016-12-20  Pedro Alves  <palves@redhat.com>
+	    Yao Qi  <yao.qi@linaro.org>
+
+	PR gdb/20977
+	* event-top.c (gdb_rl_callback_read_char_wrapper_noexcept): New
+	noexcept function, factored out from ...
+	(gdb_rl_callback_read_char_wrapper): ... this.
+	(gdb_rl_callback_handler): Mark noexcept.
+
 2016-12-20  Antoine Tremblay  <antoine.tremblay@ericsson.com>
 
 	* .dir-locals.el: Set c++ mode for the directory and set indent

gdb/event-top.c

@@ -157,10 +157,12 @@ void (*after_char_processing_hook) (void);
    sjlj-based TRY/CATCH mechanism, which knows to handle multiple
    levels of active setjmp/longjmp frames, needed in order to handle
    the readline callback recursing, as happens with e.g., secondary
-   prompts / queries, through gdb_readline_wrapper.  */
+   prompts / queries, through gdb_readline_wrapper.  This must be
+   noexcept in order to avoid problems with mixing sjlj and
+   (sjlj-based) C++ exceptions.  */
 
-static void
-gdb_rl_callback_read_char_wrapper (gdb_client_data client_data)
+static struct gdb_exception
+gdb_rl_callback_read_char_wrapper_noexcept () noexcept
 {
   struct gdb_exception gdb_expt = exception_none;
 
@@ -180,17 +182,28 @@ gdb_rl_callback_read_char_wrapper (gdb_client_data client_data)
     }
   END_CATCH_SJLJ
 
+  return gdb_expt;
+}
+
+static void
+gdb_rl_callback_read_char_wrapper (gdb_client_data client_data)
+{
+  struct gdb_exception gdb_expt
+    = gdb_rl_callback_read_char_wrapper_noexcept ();
+
   /* Rethrow using the normal EH mechanism.  */
   if (gdb_expt.reason < 0)
     throw_exception (gdb_expt);
 }
 
 /* GDB's readline callback handler.  Calls the current INPUT_HANDLER,
    and propagates GDB exceptions/errors thrown from INPUT_HANDLER back
-   across readline.  See gdb_rl_callback_read_char_wrapper.  */
+   across readline.  See gdb_rl_callback_read_char_wrapper.  This must
+   be noexcept in order to avoid problems with mixing sjlj and
+   (sjlj-based) C++ exceptions.  */
 
 static void
-gdb_rl_callback_handler (char *rl)
+gdb_rl_callback_handler (char *rl) noexcept
 {
   struct gdb_exception gdb_rl_expt = exception_none;
   struct ui *ui = current_ui;