allocate AP kernel/IST stacks at bring-up, not statically

Hoist max_cpus into danos (root.zig), raise it to 128, and stop reserving a static [max_cpus][16 KiB] IST array. Only the BSP's IST stack is static (needed before the allocator); each AP's is heap-allocated when it comes online. Shrinks the kernel from ~1.1 MB to ~139 KiB in ReleaseSmall.

src/device/acpi.zig

@@ -15,6 +15,7 @@
 //! the `Hal.mapMmio` callback the caller supplies (the arch VMM's map primitive).
 
 const std = @import("std");
+const danos = @import("danos");
 const device = @import("device.zig");
 const aml = @import("aml/aml.zig");
 const DeviceTree = device.DeviceTree;
@@ -115,7 +116,7 @@ pub const CpuInfo = struct {
     dropped: usize = 0,
 };
 
-const max_cpus = 64;
+const max_cpus = danos.max_cpus;
 
 /// Filled in by `discover` (from the MADT); SMP bring-up reads it to wake the APs.
 pub var cpu_info: CpuInfo = .{};