Put misc/gdb.py [experimental]

This works like:

```
(gdb) cfp
CFP (count=3, addr=0x7ffff73fef50):
$1 = {pc = 0x555556bf7818, sp = 0x7ffff72ff078, iseq = 0x7ffff2603270, self = 140737344619296, ep = 0x7ffff72ff058, block_code = 0x0,
  __bp__ = 0x7ffff72ff060, jit_return = 0x555558c2b000}

Stack (size=3):
[0] FIXNUM: 1
[1] T_STRING: "" bytesize:0 (embed) encoding:1 coderange:7bit $2 = (struct RString *) 0x7ffff249ea80
[2] [PROMOTED] T_OBJECT: $3 = {flags = 21474844769, klass = 140737344040416}
$4 = {0x24, 0x24, 0x24}

(gdb) cfp + 1
CFP (count=3, addr=0x7ffff73fef90):
$5 = {pc = 0x5555567a78f8, sp = 0x7ffff72ff040, iseq = 0x7ffff26032d0, self = 140737344619296, ep = 0x7ffff72ff038, block_code = 0x0,
  __bp__ = 0x7ffff72ff040, jit_return = 0x555558c2b000}

Stack (size=0):
```

1 files changed, 33 insertions, 0 deletions

diff --git a/misc/gdb.py b/misc/gdb.py
new file mode 100644
index 0000000000..038ebfc069
--- /dev/null
+++ b/misc/gdb.py
@@ -0,0 +1,33 @@
+# Usage:
+#   cfp: Dump the current cfp
+#   cfp + 1: Dump the caller cfp
+class CFP(gdb.Command):
+    def __init__(self):
+        super(CFP, self).__init__('cfp', gdb.COMMAND_USER)
+
+    def invoke(self, offset, from_tty):
+        if not offset:
+            offset = '0'
+        cfp = f'(ruby_current_ec->cfp + ({offset}))'
+
+        end_cfp = self.get_int('ruby_current_ec->vm_stack + ruby_current_ec->vm_stack_size')
+        cfp_count = int((end_cfp - self.get_int('ruby_current_ec->cfp')) / self.get_int('sizeof(rb_control_frame_t)'))
+
+        print('CFP (count={}, addr=0x{:x}):'.format(cfp_count, self.get_int(cfp)))
+        gdb.execute(f'p *({cfp})')
+        print()
+
+        stack_size = int((self.get_int(f'{cfp}->sp') - self.get_int(f'{cfp}->__bp__')) / 8)
+        print(f'Stack (size={stack_size}):')
+        for i in range(0, stack_size):
+            obj = self.get_int(f'{cfp}->__bp__[{i}]')
+            rp = self.get_string(f'rp {obj}')
+            print(f'[{i}] {rp}', end='')
+
+    def get_int(self, expr):
+        return int(self.get_string(f'printf "%ld", ({expr})'))
+
+    def get_string(self, expr):
+        return gdb.execute(expr, to_string=True)
+
+CFP()