8250824: AArch64: follow up for JDK-8248414

The original change missed to update an assert.

Backport-of: d9abf60

src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp

@@ -1496,7 +1496,7 @@ void MacroAssembler::movptr(Register r, uintptr_t imm64) {
     block_comment(buffer);
   }
 #endif
-  assert(imm64 < (1ul << 48), "48-bit overflow in address constant");
+  assert(imm64 < (1ull << 48), "48-bit overflow in address constant");
   movz(r, imm64 & 0xffff);
   imm64 >>= 16;
   movk(r, imm64 & 0xffff, 16);