rt: Zero the bottom frame's return address and base pointer

My reading of libunwind leads me to believe this is expected.

Closes #1322

Author: brson

Diff for: src/rt/arch/i386/context.cpp

@@ -31,10 +31,14 @@ void context::call(void *f, void *arg, void *stack) {
   // Shift the stack pointer so the alignment works out right.
   sp = align_down(sp) - 3;
   *--sp = (uint32_t)arg;
-  *--sp = 0xdeadbeef;
+  // The final return address. 0 indicates the bottom of the stack
+  *--sp = 0;
 
   regs.esp = (uint32_t)sp;
   regs.eip = (uint32_t)f;
+
+  // Last base pointer on the stack should be 0
+  regs.ebp = 0;
 }
 
 #if 0

Diff for: src/rt/arch/x86_64/context.cpp

@@ -28,9 +28,13 @@ void context::call(void *f, void *arg, void *stack) {
   // set up the stack
   uint64_t *sp = (uint64_t *)stack;
   sp = align_down(sp);
-  *--sp = 0xdeadbeef; // takes place of ret. addr.
+  // The final return address. 0 indicates the bottom of the stack
+  *--sp = 0;
 
   regs.data[RUSTRT_ARG0] = (uint64_t)arg;
   regs.data[RUSTRT_RSP] = (uint64_t)sp;
   regs.data[RUSTRT_IP] = (uint64_t)f;
+
+  // Last base pointer on the stack should be 0
+  regs.data[RUSTRT_RBP] = 0;
 }