idt: use constexpr for constants

Use `constexpr` for constants to prevent the compiler from generating
an external reference under all circumstances.

Fixes a build error with gcc-9 complaining that Idt::_idt_max is an
undefined reference in jdb_kern_info-ia32.o.

Change-Id: I756f0bcbb543cd20800767da121d85e385876017

src/kern/ia32/idt.cpp

@@ -18,10 +18,10 @@ class Idt
   friend class Jdb_kern_info_bench;
 
 public:
-  static const unsigned _idt_max = FIASCO_IDT_MAX;
+  static constexpr unsigned _idt_max = FIASCO_IDT_MAX;
 
 private:
-  static const Address _idt = Mem_layout::Idt;
+  static constexpr Address _idt = Mem_layout::Idt;
   static Address _idt_pa;
 };