cpu.*.bootstrap: return address for (set-context)

Instead of pushing zero as the fake return address for a set-context call, push an address that points back into the primitive so that callstack walking can match the frame back to the subprimitive.

basis/cpu/x86/32/bootstrap.factor

@@ -294,9 +294,9 @@ IN: bootstrap.x86
 
 ! Contexts
 : jit-switch-context ( reg -- )
-    ! Reset return value since its bogus right now, to avoid
-    ! confusing the GC
-    ESP -4 [+] 0 MOV
+    ! Push a bogus return address so the GC can track this frame back
+    ! to the owner
+    0 CALL
 
     ! Make the new context the current one
     ctx-reg swap MOV

basis/cpu/x86/64/bootstrap.factor

@@ -263,9 +263,9 @@ IN: bootstrap.x86
 
 ! Contexts
 : jit-switch-context ( reg -- )
-    ! Reset return value since its bogus right now, to avoid
-    ! confusing the GC
-    RSP -8 [+] 0 MOV
+    ! Push a bogus return address so the GC can track this frame back
+    ! to the owner
+    0 CALL
 
     ! Make the new context the current one
     ctx-reg swap MOV