Adds an API for setting the stack limit per-thread.

src/x64/simulator-x64.h

Index: src/x64/simulator-x64.h
diff --git a/src/x64/simulator-x64.h b/src/x64/simulator-x64.h
index 184c166bc0592a271320e7f51a851a8595fcb24d..7aa29b5c08e1b61fed8ba05ac55361c54e5f2d6a 100644
--- a/src/x64/simulator-x64.h
+++ b/src/x64/simulator-x64.h
@@ -35,15 +35,11 @@
 #define CALL_GENERATED_CODE(entry, p0, p1, p2, p3, p4) \
   entry(p0, p1, p2, p3, p4);
 
-// Calculated the stack limit beyond which we will throw stack overflow errors.
-// This macro must be called from a C++ method. It relies on being able to take
-// the address of "this" to get a value on the current execution stack and then
-// calculates the stack limit based on that value.
-// NOTE: The check for overflow is not safe as there is no guarantee that the
-// running thread has its stack in all memory up to address 0x00000000.
-#define GENERATED_CODE_STACK_LIMIT(limit) \
-  (reinterpret_cast<uintptr_t>(this) >= limit ? \
-      reinterpret_cast<uintptr_t>(this) - limit : 0)
+// The stack limit beyond which we will throw stack overflow errors in
+// generated code. Because generated code on x64 uses the C stack, we
+// just use the C stack limit.
+#define GENERATED_CODE_STACK_LIMIT(c_limit) \
+  (c_limit)
 
 // Call the generated regexp code directly. The entry function pointer should
 // expect seven int/pointer sized arguments and return an int.