panic: print backtrace on panic

Panics currently lack backtraces, which are a very useful thing to
have. Make the profiling backtrace function generic and print backtraces
using it.

kernel-src/include/x86-64/arch/backtrace.h

@@ -0,0 +1,11 @@
+#ifndef _ARCH_BACKTRACE_H
+#define _ARCH_BACKTRACE_H
+
+#include <stdbool.h>
+#include <arch/context.h>
+
+typedef bool (*backtrace_fn_t)(void *ctx, void *ip);
+
+void arch_get_backtrace(context_t *cpu_context, backtrace_fn_t fn, void *context);
+
+#endif