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

runtime/vm/simulator_arm.cc

Index: runtime/vm/simulator_arm.cc
diff --git a/runtime/vm/simulator_arm.cc b/runtime/vm/simulator_arm.cc
index f11bab757161a53db18f63a373a7e44959d6a546..24c934b54766d2d50e790806f70b6f9389b6d253 100644
--- a/runtime/vm/simulator_arm.cc
+++ b/runtime/vm/simulator_arm.cc
@@ -3759,7 +3759,7 @@ int64_t Simulator::Call(int32_t entry,
   int32_t r6_val = get_register(R6);
   int32_t r7_val = get_register(R7);
   int32_t r8_val = get_register(R8);
-#if !defined(TARGET_OS_MACOS)
+#if defined(TARGET_ABI_EABI)
   int32_t r9_val = get_register(R9);
 #endif
   int32_t r10_val = get_register(R10);
@@ -3793,7 +3793,7 @@ int64_t Simulator::Call(int32_t entry,
   set_register(R6, callee_saved_value);
   set_register(R7, callee_saved_value);
   set_register(R8, callee_saved_value);
-#if !defined(TARGET_OS_MACOS)
+#if defined(TARGET_ABI_EABI)
   set_register(R9, callee_saved_value);
 #endif
   set_register(R10, callee_saved_value);
@@ -3821,7 +3821,7 @@ int64_t Simulator::Call(int32_t entry,
   ASSERT(callee_saved_value == get_register(R6));
   ASSERT(callee_saved_value == get_register(R7));
   ASSERT(callee_saved_value == get_register(R8));
-#if !defined(TARGET_OS_MACOS)
+#if defined(TARGET_ABI_EABI)
   ASSERT(callee_saved_value == get_register(R9));
 #endif
   ASSERT(callee_saved_value == get_register(R10));
@@ -3844,7 +3844,7 @@ int64_t Simulator::Call(int32_t entry,
   set_register(R6, r6_val);
   set_register(R7, r7_val);
   set_register(R8, r8_val);
-#if !defined(TARGET_OS_MACOS)
+#if defined(TARGET_ABI_EABI)
   set_register(R9, r9_val);
 #endif
   set_register(R10, r10_val);