Support 64 bit return value for stub (#1034)

The `return (int) X` command in gdb only sets the 4 lowest bytes of the
return register. For example if `rax` was `-1`, a `return int 0` would
leave us with `0xffffffff000000` instead of `0x0`.

This patch makes `StubBreakpoint` check for the register size and
executes `return (int)` or `return (long)` accordingly.

gef.py

@@ -4147,7 +4147,8 @@ def __init__(self, func: str, retval: Optional[int]) -> None:
         return
 
     def stop(self) -> bool:
-        gdb.execute(f"return (unsigned int){self.retval:#x}")
+        size = "long" if gef.arch.ptrsize == 8 else "int"
+        gdb.execute(f"return (unsigned {size}){self.retval:#x}")
         ok(f"Ignoring call to '{self.func}' "
            f"(setting return value to {self.retval:#x})")
         return False