[BOLT][AArch64] Implemented createDummyReturnFunction.

On AArch64, this method is needed when trying to instrument a static
binary.

Sample commands:
```bash
clang -Wl,-q test.c -static -o out
llvm-bolt -instrument -instrumentation-sleep-time=5 out -o out.instr
```

Author: paschalis-mpeis

bolt/lib/Target/AArch64/AArch64MCPlusBuilder.cpp

@@ -1607,6 +1607,12 @@ class AArch64MCPlusBuilder : public MCPlusBuilder {
     return Instrs;
   }
 
+  InstructionListType createDummyReturnFunction(MCContext *Ctx) const override {
+    InstructionListType Insts(1);
+    createReturn(Insts[0]);
+    return Insts;
+  }
+
   std::vector<MCInst> createSymbolTrampoline(const MCSymbol *TgtSym,
                                              MCContext *Ctx) override {
     std::vector<MCInst> Insts;

bolt/test/AArch64/dummy-return.test

@@ -1,6 +1,10 @@
+// Tests that AArch64 is able to instrument static binaries in relocation mode.
+
 REQUIRES: system-linux
 
 RUN: %clang %p/../Inputs/main.c -o %t -Wl,-q -static
-RUN: not llvm-bolt -instrument -instrumentation-sleep-time=1 %t -o %t.instr 2>&1 | FileCheck %s
+RUN: llvm-bolt -instrument -instrumentation-sleep-time=1 %t -o %t.instr 2>&1 | FileCheck %s
+RUN: llvm-nm -n %t.instr | FileCheck %s -check-prefix=CHECK-SYM
 
-CHECK: not implemented
+CHECK: BOLT-INFO: output linked against instrumentation runtime library
+CHECK-SYM: __bolt_fini_trampoline