switch to GCC 11

According to [issue mit-pdos#125 ](mit-pdos#125),
 the problem was found in GCC 11, so switch to GCC 11

Makefile

@@ -65,7 +65,7 @@ CFLAGS += $(shell $(CC) -fno-stack-protector -E -x c /dev/null >/dev/null 2>&1 &
 
 # Enable flag -Wno-error=infinite-recursion when available
 GCC_VERSION = $(shell $(CC) -dumpversion | cut -d '.' -f1)
-ifeq ($(shell echo $(GCC_VERSION) / 12 2>/dev/null | bc | grep -e '^0'), )
+ifeq ($(shell echo $(GCC_VERSION) / 11 2>/dev/null | bc | grep -e '^0'), )
 CFLAGS += -Wno-error=infinite-recursion
 endif
 # Disable PIE when possible (for Ubuntu 16.10 toolchain)