Fix the gdb configuration

src/utils/pintos-gdb

 #! /bin/sh
 
 # Path to GDB macros file.  Customize for your site.
-GDBMACROS=$HOME/pintos/src/misc/gdb-macros
+GDBMACROS=$PINTOS_HOME/src/misc/gdb-macros
 
 # Choose correct GDB.
 if command -v i386-elf-gdb >/dev/null 2>&1; then