Allocate the right number of fast context slots on X64 and ARM. Port from ia32.

This is ported from change 3505 on ia32.
Review URL: http://codereview.chromium.org/1992003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4603 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/arm/codegen-arm.cc

@@ -191,7 +191,7 @@ void CodeGenerator::Generate(CompilationInfo* info) {
       frame_->AllocateStackSlots();
 
       VirtualFrame::SpilledScope spilled_scope(frame_);
-      int heap_slots = scope()->num_heap_slots();
+      int heap_slots = scope()->num_heap_slots() - Context::MIN_CONTEXT_SLOTS;
       if (heap_slots > 0) {
         // Allocate local context.
         // Get outer context and create a new context based on it.

src/x64/codegen-x64.cc

@@ -360,7 +360,7 @@ void CodeGenerator::Generate(CompilationInfo* info) {
       frame_->AllocateStackSlots();
 
       // Allocate the local context if needed.
-      int heap_slots = scope()->num_heap_slots();
+      int heap_slots = scope()->num_heap_slots() - Context::MIN_CONTEXT_SLOTS;
       if (heap_slots > 0) {
         Comment cmnt(masm_, "[ allocate local context");
         // Allocate local context.