[arm64] Fix GCC build warnings.

Remove a couple of variables that are written but not read.

BUG=

Review-Url: https://codereview.chromium.org/1923913002
Cr-Commit-Position: refs/heads/master@{#35847}

src/arm64/builtins-arm64.cc

@@ -1194,7 +1194,6 @@ void Builtins::Generate_CompileLazy(MacroAssembler* masm) {
   Label loop_top, loop_bottom;
 
   Register closure = x1;
-  Register new_target = x3;
   Register map = x13;
   Register index = x2;
   __ Ldr(map, FieldMemOperand(closure, JSFunction::kSharedFunctionInfoOffset));

src/compiler/arm64/instruction-selector-arm64.cc

@@ -2249,19 +2249,15 @@ void InstructionSelector::VisitAtomicLoad(Node* node) {
   Node* base = node->InputAt(0);
   Node* index = node->InputAt(1);
   ArchOpcode opcode = kArchNop;
-  ImmediateMode immediate_mode = kNoImmediate;
   switch (load_rep.representation()) {
     case MachineRepresentation::kWord8:
       opcode = load_rep.IsSigned() ? kAtomicLoadInt8 : kAtomicLoadUint8;
-      immediate_mode = kLoadStoreImm8;
       break;
     case MachineRepresentation::kWord16:
       opcode = load_rep.IsSigned() ? kAtomicLoadInt16 : kAtomicLoadUint16;
-      immediate_mode = kLoadStoreImm16;
       break;
     case MachineRepresentation::kWord32:
       opcode = kAtomicLoadWord32;
-      immediate_mode = kLoadStoreImm32;
       break;
     default:
       UNREACHABLE();