[kernel] Fix incorrect BSP idle rsp0

In bsp_early_init(), the BSP cpu_data's rsp0 was getting initialized to
the _value_ at the idle_stack_end symbol, instead of its address. I
don't believe this was causing any actual harm, but it was a red herring
when debugging.

src/kernel/cpu.cpp

@@ -85,7 +85,7 @@ bsp_early_init()
     cpu->idt = new (&g_bsp_idt) IDT;
     cpu->tss = new (&g_bsp_tss) TSS;
     cpu->gdt = new (&g_bsp_gdt) GDT {cpu->tss};
-    cpu->rsp0 = idle_stack_end;
+    cpu->rsp0 = reinterpret_cast<uintptr_t>(&idle_stack_end);
     cpu_early_init(cpu);
 
     return cpu;