Propagate GDB/C++ exceptions across readline using sj/lj-based TRY/CATCH

If we map GDB'S TRY/CATCH macros to C++ try/catch, GDB breaks on
systems where readline isn't built with exceptions support.  The
problem is that readline calls into GDB through the callback
interface, and if GDB's callback throws a C++ exception/error, the
system unwinder won't manage to unwind past the readline frame, and
ends up calling std::terminate(), which aborts the process:

 (gdb) whatever-command-that-causes-an-error
 terminate called after throwing an instance of 'gdb_exception_RETURN_MASK_ERROR'
 Aborted
 $

This went unnoticed for so long because:

- the x86-64 ABI requires -fasynchronous-unwind-tables, making it
  possible for exceptions to cross readline with no special handling.
  But e.g., on ARM or AIX, unless you build readline with
  -fexceptions, you trip on the problem.

- TRY/CATCH was mapped to setjmp/longjmp, even in C++ mode, until
  quite recently.

The fix is to catch and save any GDB exception that is thrown inside
the GDB readline callback, and then once the callback returns back to
the GDB code that called into readline in the first place, rethrow the
saved GDB exception.

This is similar in spirit to how we catch/map GDB exceptions at the
GDB/Python and GDB/Guile API boundaries.

The next question is then: if we intercept all exceptions within GDB's
readline callback, should we simply return normally to readline?  The
callback prototype has no way to signal an error back to readline (*).
The answer is no -- if we return normally, we'll be returning to a
loop inside rl_callback_read_char that continues processing pending
input, calling into GDB again, redisplaying the prompt, etc.  Thus if
we want to error out of rl_callback_read_char, we need to long jump
across it, just like we always did before TRY/CATCH were ever mapped
to C++ exceptions.

My first approach built a specialized API to handle this, with a
couple macros to hide the setjmp/longjmp and the struct gdb_exception
saving/rethrowing.

However, I realized that we need to:

 - Handle multiple active rl_callback_read_char invocations.  If,
   while processing input something triggers a secondary prompt, we
   end up in a nested rl_callback_read_char call, through
   gdb_readline_wrapper.

 - Propagate a struct gdb_exception along with the longjmp.

... and that this is exactly what the setjmp/longjmp-based TRY/CATCH
does.

So the fix makes the setjmp/longjmp TRY/CATCH always available under
new TRY_SJLJ/CATCH_SJLJ aliases, even when TRY/CATCH is mapped to C++
try/catch, and then uses TRY_SJLJ/CATCH_SJLJ to propagate GDB
exceptions across the readline callback.

This turns out to be a much better looking fix than my bespoke API
attempt, even.  We'll probably be able to simplify TRY_SJLJ/CATCH_SJLJ
when we finally get rid of TRY/CATCH all over the tree, but until
then, this reuse seems quite nice for avoiding a second parallel
setjmp/longjmp mechanism.

(*) - maybe we could propose a readline API change, but we still need
      to handle current readline, anyway.

gdb/ChangeLog:
2016-04-22  Pedro Alves  <palves@redhat.com>

	* common/common-exceptions.c (enum catcher_state, struct catcher)
	(current_catcher): Define in C++ mode too.
	(exceptions_state_mc_catch): Call throw_exception_sjlj instead of
	throw_exception.
	(throw_exception_sjlj, throw_exception_cxx): New functions,
	factored out from throw_exception.
	(throw_exception): Reimplement.
	* common/common-exceptions.h (exceptions_state_mc_init)
	(exceptions_state_mc_action_iter)
	(exceptions_state_mc_action_iter_1, exceptions_state_mc_catch):
	Declare in C++ mode too.
	(TRY): Rename to ...
	(TRY_SJLJ): ... this.
	(CATCH): Rename to ...
	(CATCH_SJLJ): ... this.
	(END_CATCH): Rename to ...
	(END_CATCH_SJLJ): ... this.
	[GDB_XCPT == GDB_XCPT_SJMP] (TRY, CATCH, END_CATCH): Map to SJLJ
	equivalents.
	(throw_exception): Update comments.
	(throw_exception_sjlj): Declare.
	* event-top.c (gdb_rl_callback_read_char_wrapper): Extend intro
	comment.  Wrap body in TRY_SJLJ/CATCH_SJLJ and rethrow any
	intercepted exception.
	(gdb_rl_callback_handler): New function.
	(gdb_rl_callback_handler_install): Always install
	gdb_rl_callback_handler as readline callback.

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