x86: enable interrupts explicitly in __start_xen()

Instead of relying on smp_prepare_cpus() (via check_nmi_watchdog()) or init_xen_time() (via init_platform_timer() -> plt_overflow()) implicitly enabling interrupts, enable them explicitly once safe to do so (it may actually be possible to move this even further up, but I don't think that would buy us much).

@@ -941,6 +941,9 @@ void __init __start_xen(unsigned long mb
         set_in_cr4(X86_CR4_OSFXSR);
     if ( cpu_has_xmm )
         set_in_cr4(X86_CR4_OSXMMEXCPT);
+
+    local_irq_enable();
+
 #ifdef CONFIG_X86_64
     vesa_mtrr_init();
 #endif