[arch][m68k] initial port to m68k

Uses the QEMU virt machine for 68k defined in qemu 6.0+.
Basic support that boots, prints to the console, takes input from
console, and context switches.

TODO: interrupt support, timer support.

platform/qemu-virt-m68k/goldfish_rtc.c

@@ -0,0 +1,65 @@
+/*
+ * Copyright (c) 2021 Travis Geiselbrecht
+ *
+ * Use of this source code is governed by a MIT-style
+ * license that can be found in the LICENSE file or at
+ * https://opensource.org/licenses/MIT
+ */
+#include "platform_p.h"
+
+#include <assert.h>
+#include <lk/err.h>
+#include <lk/debug.h>
+#include <lk/reg.h>
+#include <lk/trace.h>
+#include <kernel/debug.h>
+#include <kernel/thread.h>
+#include <platform/interrupts.h>
+#include <platform/virt.h>
+#include <platform/timer.h>
+
+#define LOCAL_TRACE 0
+
+static platform_timer_callback t_callback;
+
+static volatile uint ticks = 0;
+static lk_time_t periodic_interval;
+
+void goldfish_rtc_early_init(void) {
+}
+
+void goldfish_rtc_init(void) {
+}
+
+lk_bigtime_t current_time_hires(void) {
+    static lk_bigtime_t bt = 0;
+    return ++bt;
+}
+
+lk_time_t current_time(void) {
+    static lk_time_t time = 0;
+    return ++time;
+}
+
+status_t platform_set_periodic_timer(platform_timer_callback callback, void *arg, lk_time_t interval) {
+    LTRACEF("callback %p, arg %p, interval %u\n", callback, arg, interval);
+
+    t_callback = callback;
+
+    periodic_interval = interval;
+
+#if 0
+    uint32_t ticks = periodic_interval * 1000; /* timer is running close to 1Mhz */
+    ASSERT(ticks <= 0xffff);
+
+    TIMREG(IEN(0)) = (1<<0); // interval interrupt
+    TIMREG(INTERVAL_VAL(0)) = ticks;
+    TIMREG(CNT_CTRL(0)) = (1<<5) | (1<<4) | (1<<1); // no wave, reset, interval mode
+
+    unmask_interrupt(TTC0_A_INT);
+#endif
+
+    return NO_ERROR;
+}
+
+