Add new overload of gdbarch_return_value

The gdbarch "return_value" can't correctly handle variably-sized
types.  The problem here is that the TYPE_LENGTH of such a type is 0,
until the type is resolved, which requires reading memory.  However,
gdbarch_return_value only accepts a buffer as an out parameter.

Fixing this requires letting the implementation of the gdbarch method
resolve the type and return a value -- that is, both the contents and
the new type.

After an attempt at this, I realized I wouldn't be able to correctly
update all implementations (there are ~80) of this method.  So,
instead, this patch adds a new method that falls back to the current
method, and it updates gdb to only call the new method.  This way it's
possible to incrementally convert the architectures that I am able to
test.

gdb/arch-utils.c

@@ -1098,6 +1098,24 @@ default_get_return_buf_addr (struct type *val_type, frame_info_ptr cur_frame)
   return 0;
 }
 
+enum return_value_convention
+default_gdbarch_return_value
+     (struct gdbarch *gdbarch, struct value *function, struct type *valtype,
+      struct regcache *regcache, struct value **read_value,
+      const gdb_byte *writebuf)
+{
+  gdb_byte *readbuf = nullptr;
+
+  if (read_value != nullptr)
+    {
+      *read_value = allocate_value (valtype);
+      readbuf = value_contents_raw (*read_value).data ();
+    }
+
+  return gdbarch_return_value (gdbarch, function, valtype, regcache,
+			       readbuf, writebuf);
+}
+
 /* Non-zero if we want to trace architecture code.  */
 
 #ifndef GDBARCH_DEBUG