[tools] Add the j6bt command to gdb functions

Added the command j6bt (and renamed popc_stack to j6stack) to make
tracking kernel callstacks across interrupts easier.

assets/debugging/jsix.elf-gdb.py

@@ -2,7 +2,7 @@ import gdb
 
 class PrintStackCommand(gdb.Command):
     def __init__(self):
-        super().__init__("popc_stack", gdb.COMMAND_DATA)
+        super().__init__("j6stack", gdb.COMMAND_DATA)
 
     def invoke(self, arg, from_tty):
         args = gdb.string_to_argv(arg)
@@ -22,7 +22,38 @@ class PrintStackCommand(gdb.Command):
             print("{:016x} (+{:04x}): {:016x}".format(int(base_addr) + offset, offset, int(value)))
 
 
+class PrintBacktraceCommand(gdb.Command):
+    def __init__(self):
+        super().__init__("j6bt", gdb.COMMAND_DATA)
+
+    def invoke(self, arg, from_tty):
+        args = gdb.string_to_argv(arg)
+
+        frame = "$rbp"
+        if len(args) > 0:
+            frame = args[0]
+
+        depth = 30
+        if len(args) > 1:
+            depth = int(args[1])
+
+        for i in range(depth-1, -1, -1):
+            ret = gdb.parse_and_eval(f"*(uint64_t*)({frame} + 8)")
+            frame = gdb.parse_and_eval(f"*(uint64_t*)({frame})")
+
+            name = ""
+            block = gdb.block_for_pc(int(ret))
+            if block:
+                name = block.function or ""
+
+            print("{:016x} {}".format(int(ret), name))
+
+            if frame == 0 or ret == 0:
+                return
+
+
 PrintStackCommand()
+PrintBacktraceCommand()
 
 gdb.execute("target remote :1234")
 gdb.execute("display/i $rip")