back.pysim: override ResetSynchronizer implementation.

This was rewritten to use Yosys cells in 779f3ee9 to avoid leaking
the interior clock domain, but the simulator doesn't understand Yosys
cells. So, use the old implementation in the simulator.

nmigen/back/pysim.py

@@ -10,6 +10,8 @@ from ..tools import flatten
 from ..hdl.ast import *
 from ..hdl.ir import *
 from ..hdl.xfrm import ValueVisitor, StatementVisitor
+from ..hdl.dsl import Module
+from ..hdl.cd import ClockDomain
 
 
 __all__ = ["Simulator", "Delay", "Tick", "Passive", "DeadlineError"]
@@ -351,9 +353,23 @@ class _StatementCompiler(StatementVisitor):
         return run
 
 
+class _SimulatorPlatform:
+    def get_reset_sync(self, reset_sync):
+        m = Module()
+        m.domains += ClockDomain("_reset_sync", async_reset=True)
+        for i, o in zip((0, *reset_sync._regs), reset_sync._regs):
+            m.d._reset_sync += o.eq(i)
+        m.d.comb += [
+            ClockSignal("_reset_sync").eq(ClockSignal(reset_sync.domain)),
+            ResetSignal("_reset_sync").eq(reset_sync.arst),
+            ResetSignal(reset_sync.domain).eq(reset_sync._regs[-1])
+        ]
+        return m
+
+
 class Simulator:
     def __init__(self, fragment, vcd_file=None, gtkw_file=None, traces=()):
-        self._fragment        = Fragment.get(fragment, platform=None)
+        self._fragment        = Fragment.get(fragment, platform=_SimulatorPlatform())
 
         self._signal_slots    = SignalDict()  # Signal -> int/slot
         self._slot_signals    = list()        # int/slot -> Signal