libgcc: Avoid warnings on __gcc_nested_func_ptr_created [PR113402]

I'm seeing hundreds of
In file included from ../../../libgcc/libgcc2.c:56:
../../../libgcc/libgcc2.h:32:13: warning: conflicting types for built-in function ‘__gcc_nested_func_ptr_created’; expected ‘void(void *, void *, void *)’
+[-Wbuiltin-declaration-mismatch]
   32 | extern void __gcc_nested_func_ptr_created (void *, void *, void **);
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
warnings.

Either we need to add like in r14-6218
  #pragma GCC diagnostic ignored "-Wbuiltin-declaration-mismatch"
(but in that case because of the libgcc2.h prototype (why is it there?)
it would need to be also with #pragma GCC diagnostic push/pop around),
or we could go with just following how the builtins are prototyped on the
compiler side and only cast to void ** when dereferencing (which is in
a single spot in each TU).

2024-02-01  Jakub Jelinek  <jakub@redhat.com>

	PR libgcc/113402
	* libgcc2.h (__gcc_nested_func_ptr_created): Change type of last
	argument from void ** to void *.
	* config/i386/heap-trampoline.c (__gcc_nested_func_ptr_created):
	Change type of dst from void ** to void * and cast dst to void **
	before dereferencing it.
	* config/aarch64/heap-trampoline.c (__gcc_nested_func_ptr_created):
	Likewise.