Fix handout code.

Author: kohler

fundamentals5x/GNUmakefile

@@ -1,6 +1,9 @@
 PROGRAMS = nogcbench gcbench
 all: $(PROGRAMS)
 
+# More garbage gets cleaned up more reliably when optimization is off.
+O ?= 0
+
 include ../common/rules.mk
 
 %.o: %.c $(BUILDSTAMP)

fundamentals5x/m61gc.c

@@ -112,7 +112,7 @@ void m61_gc(void) {
     __asm__ __volatile__("" : : : "rbx", "r12", "r13", "r14", "r15", "memory");
 #endif
 
-    char* stack_top = (char*) __builtin_frame_address(0);
+    char* stack_top = (char*) &stack_top;
 
     // unmark all active allocations
     // YOUR CODE HERE