[kernel] Pre-allocate cpu_data and pass to APs

In order to avoid cyclic dependencies in the case of page faults while
bringing up an AP, pre-allocate the cpu_data structure and related CPU
control structures, and pass them to the AP startup code.

This also changes the following:
- cpu_early_init() was split out of cpu_early_init() to allow early
  usage of current_cpu() on the BSP before we're ready for the rest of
  cpu_init(). (These functions were also renamed to follow the preferred
  area_action naming style.)
- isr_handler now zeroes out the IST entry for its vector instead of
  trying to increment the IST stack pointer
- the IST stacks are allocated outside of cpu_init, to also help reduce
  stack pressue and chance of page faults before APs are ready
- share stack areas between AP idle threads so we only waste 1K per
  additional AP for the unused idle stack

src/kernel/tss.h

@@ -22,6 +22,10 @@ public:
 	/// \returns   A mutable reference to the stack pointer
 	uintptr_t & ist_stack(unsigned ist);
 
+	/// Allocate new stacks for the given IST entries.
+	/// \arg ist_entries  A bitmap of used IST entries
+	void create_ist_stacks(uint8_t ist_entries);
+
 private:
 	uint32_t m_reserved0;