[tools] Default to printing $pc when jco is called without args

This makes jco on gdb behave the same as jco on lldb.

Bug: v8:11879
Change-Id: Id6a338878d518984986d2b719588966ee09de3c6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3000956Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75620}

tools/gdbinit

@@ -31,7 +31,11 @@ end
 
 # Print Code objects containing given PC.
 define jco
-call (void) _v8_internal_Print_Code((void*)($arg0))
+  if $argc == 0
+    call (void) _v8_internal_Print_Code((void*)($pc))
+  else
+    call (void) _v8_internal_Print_Code((void*)($arg0))
+  end
 end
 document jco
 Print a v8 Code object from an internal code address