Aarch64: Fix segfault when casting dummy calls

Aarch64: Fix segfault when casting dummy calls

The following will segfault on aarch64 if foo is in another object,
was compiled as c++ and has no debug symbols:
(gdb) p (int)foo()

This is because aarch64_push_dummy_call determines the return type
of the function and then does not check for null pointer.

A null pointer for the return type means the call has no debug
information.  For the code to get here, then the call must have
been cast, otherwise we'd error out sooner.  In the case of a
no-debug-info call cast, the return type is the type the user
had cast the call to, but we do not have that information
available here.

However, aarch64_push_dummy_call only requires the return type in
order to calculate lang_struct_return. This information is available
in the return_method enum. The fix is to simply use this instead.

Adds testcase to check calls across objects, with all combinations
of c, c++, debug and no debug.

gdb/ChangeLog:

	PR gdb/22736:
	* aarch64-tdep.c (aarch64_push_dummy_call): Remove
	lang_struct_return code.

gdb/testsuite/ChangeLog:

	PR gdb/22736:
	* gdb.cp/infcall-nodebug-lib.c: New test.
	* gdb.cp/infcall-nodebug-main.c: New test.
	* gdb.cp/infcall-nodebug.exp: New file.