gdb: mark disassembler function callback types as noexcept

gdb: mark disassembler function callback types as noexcept

In disasm.h we define a set of types that are used by the various
disassembler classes to hold callback functions before passing the
callbacks into libopcodes.

Because libopcodes is C code, and on some (many?) targets, C code is
compiled without exception support, it is important that GDB not try
to throw an exception over libopcode code.

In the previous commit all the existing callbacks were marked as
noexcept, however, this doesn't protect us from a future change to GDB
either adding a new callback that is not noexcept, or removing the
noexcept keyword from an existing callback.

In this commit I mark all the callback types as noexcept.  This means
that GDB's disassembler classes will no longer compile if we try to
pass a callback that is not marked as noexcept.

At least, that's the idea.  Unfortunately, it's not that easy.

Before C++17, the noexcept keyword on a function typedef would be
ignored, thus:

  using func_type = void (*) (void) noexcept;

  void
  a_func ()
  {
    throw 123;
  }

  void
  some_func (func_type f)
  {
    f ();
  }

  int
  main ()
  {
    some_func (a_func);
    return 0;
  }

Will compile just fine for C++11 and C++14 with GCC.  Clang on the
other hand complains that 'noexcept' should not appear on function
types, but then does appear to correctly complain that passing a_func
is a mismatch in the set of exceptions that could be thrown.

Switching to C++17 and both GCC and Clang correctly point out that
passing a_func is an invalid conversion relating to the noexcept
keyword.  Changing a_func to:

  void
  a_func () noexcept
  { /* Nothing.  */ }

And for C++17 both GCC and Clang compile this just fine.

My conclusion then is that adding the noexcept keyword to the function
types is pointless while GDB is not compiled as C++17, and silencing
the warnings would require us to jump through a bunch of hoops.

And so, in this commit, I define a macro LIBOPCODE_CALLBACK_NOEXCEPT,
this macro expands to noexcept when compiling for C++17, but otherwise
expands to nothing.  I then add this macro to the function types.

I've compiled GDB as the default C++11 and also forced the compile to
C++17.  When compiling as C++17 I spotted a few additional places
where callbacks needed to be marked noexcept (these fixes were merged
into the previous commit, but this confirmed to be that the macro is
working as expected).