[arm/sim] Add 'sim' gdb command and backtrace command

Port the recent arm64 simulator debugger improvements to arm:

  * [arm64/sim] Add a 'sim' gdb command
    https://crrev.com/c/2664448
  * [arm64/sim] Add a simple backtrace debug command
    https://crrev.com/c/2666688

Change-Id: Id58a49cfe8e643be0fb6718a0a149e2c7ea0bcaa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2844659
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74123}

src/execution/arm/simulator-arm.h

@@ -77,6 +77,7 @@ class Simulator : public SimulatorBase {
     r14,
     r15,
     num_registers,
+    fp = 11,
     sp = 13,
     lr = 14,
     pc = 15,
@@ -255,8 +256,10 @@ class Simulator : public SimulatorBase {
   uintptr_t PopAddress();
 
   // Debugger input.
-  void set_last_debugger_input(char* input);
-  char* last_debugger_input() { return last_debugger_input_; }
+  void set_last_debugger_input(ArrayUniquePtr<char> input) {
+    last_debugger_input_ = std::move(input);
+  }
+  const char* last_debugger_input() { return last_debugger_input_.get(); }
 
   // Redirection support.
   static void SetRedirectInstruction(Instruction* instruction);
@@ -468,7 +471,7 @@ class Simulator : public SimulatorBase {
   int icount_;
 
   // Debugger input.
-  char* last_debugger_input_;
+  ArrayUniquePtr<char> last_debugger_input_;
 
   // Registered breakpoints.
   Instruction* break_pc_;