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/arch/x86-64/backtrace.c

@@ -0,0 +1,53 @@
+#include <arch/backtrace.h>
+#include <stdint.h>
+
+static bool valid_rip(void *rip) {
+	if ((uint64_t)rip < (uint64_t)0xffffffff80000000)
+		return false;
+
+	return true;
+}
+
+// TODO 5-level paging support?
+static bool valid_frame(void *frame, void *old_frame) {
+	if ((uint64_t)frame < (uint64_t)0xffff800000000000)
+		return false;
+
+	if ((uint64_t)frame >= (uint64_t)0xffffffff80000000)
+		return false;
+
+	if ((uint64_t)old_frame >= (uint64_t)frame)
+		return false;
+
+	return true;
+}
+
+void arch_get_backtrace(context_t *cpu_context, backtrace_fn_t fn, void *context) {
+	if (!valid_rip((void *)cpu_context->rip))
+		return;
+
+	if (!fn(context, (void *)cpu_context->rip))
+		return;
+
+	size_t loops = 0;
+	uint64_t *rbp = (uint64_t *)cpu_context->rbp;
+	uint64_t *old_frame = NULL;
+	for (;;) {
+		if (!valid_frame(rbp, old_frame))
+			return;
+
+		void *rip = (void *)*(rbp + 1);
+		if (!valid_rip(rip))
+			return;
+
+		if (!fn(context, rip))
+			return;
+
+		old_frame = rbp;
+		rbp = (uint64_t *)*rbp;
+
+		if (++loops == 256)
+			return;
+	}
+}
+