Don't assume we want the iOS ABI if running simarm on Mac and the EABI otherwise.

runtime/vm/os_macos.cc

Index: runtime/vm/os_macos.cc
diff --git a/runtime/vm/os_macos.cc b/runtime/vm/os_macos.cc
index 6d2d02ac1ffdb59bd3233aa9023d7d43e465a786..e231e5ccbc100f16f368468b12534c571d013a16 100644
--- a/runtime/vm/os_macos.cc
+++ b/runtime/vm/os_macos.cc
@@ -204,8 +204,24 @@ intptr_t OS::ActivationFrameAlignment() {
 
 
 intptr_t OS::PreferredCodeAlignment() {
-  ASSERT(32 <= OS::kMaxPreferredCodeAlignment);
-  return 32;
+#if defined(TARGET_ARCH_IA32) ||                                               \
+    defined(TARGET_ARCH_X64) ||                                                \
+    defined(TARGET_ARCH_ARM64) ||                                              \
+    defined(TARGET_ARCH_DBC)
+  const int kMinimumAlignment = 32;
+#elif defined(TARGET_ARCH_ARM) || defined(TARGET_ARCH_MIPS)
+  const int kMinimumAlignment = 16;
+#else
+#error Unsupported architecture.
+#endif
+  intptr_t alignment = kMinimumAlignment;
+  // TODO(5411554): Allow overriding default code alignment for
+  // testing purposes.
+  // Flags::DebugIsInt("codealign", &alignment);
+  ASSERT(Utils::IsPowerOfTwo(alignment));
+  ASSERT(alignment >= kMinimumAlignment);
+  ASSERT(alignment <= OS::kMaxPreferredCodeAlignment);
+  return alignment;
 }