[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/cpu.h

@@ -38,9 +38,16 @@ struct cpu_data
 
 extern "C" cpu_data * _current_gsbase();
 
-/// Initialize a CPU and set up its cpu_data structure
-/// \arg bsp  True if the current CPU is the BSP
-void init_cpu(bool bsp);
+/// Set up the running CPU. This sets GDT, IDT, and necessary MSRs as well as creating
+/// the cpu_data structure for this processor.
+/// \arg cpu  The cpu_data structure for this CPU
+/// \arg bsp  True if this CPU is the BSP
+void cpu_init(cpu_data *cpu, bool bsp);
+
+/// Do early (before cpu_init) initialization work. Only needs to be called manually for
+/// the BSP, otherwise cpu_init will call it.
+/// \arg cpu  The cpu_data structure for this CPU
+void cpu_early_init(cpu_data *cpu);
 
 /// Get the cpu_data struct for the current executing CPU
 inline cpu_data & current_cpu() { return *_current_gsbase(); }
@@ -49,7 +56,3 @@ inline cpu_data & current_cpu() { return *_current_gsbase(); }
 /// validated the required features, but still iterate the options and log about them.
 void cpu_validate();
 
-/// Set up the running CPU. This sets GDT, IDT, and necessary MSRs as well as creating
-/// the cpu_data structure for this processor.
-/// \arg bsp  True if this CPU is the BSP
-void cpu_initialize(bool bsp);