[vm] Avoid clobbering array length in AllocateArray stub on ARM

It was calling EnsureIsNewOrRemembered on the slow path which
was forgetting to preserve registers around a runtime call.

Fixes https://github.com/flutter/flutter/issues/106510

TEST=vm/dart{,_2}/flutter_regress_106510

Cq-Include-Trybots: luci.dart.try:vm-kernel-linux-release-simarm-try,vm-kernel-precomp-linux-release-simarm-try
Change-Id: I621e392304fcd1fd643c009fbcde3f88b6f19b7f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/250168
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>

runtime/vm/compiler/stub_code_compiler_arm.cc

@@ -46,7 +46,7 @@ void StubCodeCompiler::EnsureIsNewOrRemembered(Assembler* assembler,
   {
     LeafRuntimeScope rt(assembler,
                         /*frame_size=*/0,
-                        /*preserve_registers=*/false);
+                        /*preserve_registers=*/preserve_registers);
     // [R0] already contains first argument.
     __ mov(R1, Operand(THR));
     rt.Call(kEnsureRememberedAndMarkingDeferredRuntimeEntry, 2);