build/parser: support set_default(cpu_type="xxx")

Ensures args_fill() for new cpu_type is called, allowing for patterns
in a target file like:

    parser.set_defaults(cpu_type="vexriscv_smp")
    parser.set_defaults(cpu_variant="linux")
    parser.set_defaults(with_fpu=True)

litex/build/parser.py

@@ -217,7 +217,8 @@ def parse_args(self, args=None, namespace=None):
                 self._platform.fill_args(self._toolchain, self)
 
         # Intercept selected CPU to fill arguments.
-        cpu_cls = cpu.CPUS.get(self.get_value_from_key("--cpu-type"), None)
+        default_cpu_type = self._args_default.get("cpu_type", None)
+        cpu_cls = cpu.CPUS.get(self.get_value_from_key("--cpu-type", default_cpu_type))
         if cpu_cls is not None and hasattr(cpu_cls, "args_fill"):
             cpu_cls.args_fill(self)