Set TARGET_OS_* from GN, falling back to HOST_OS_*.

runtime/vm/stub_code_arm.cc

Index: runtime/vm/stub_code_arm.cc
diff --git a/runtime/vm/stub_code_arm.cc b/runtime/vm/stub_code_arm.cc
index 982ec58071949ce3067328e2bd20a1811fb2aaec..ca47798c0ad14502a3bd7ed9da765ab3a812d817 100644
--- a/runtime/vm/stub_code_arm.cc
+++ b/runtime/vm/stub_code_arm.cc
@@ -814,9 +814,9 @@ void StubCode::GenerateInvokeDartCodeStub(Assembler* assembler) {
 
   // kExitLinkSlotFromEntryFp must be kept in sync with the code below.
   __ Push(R4);
-#if defined(TARGET_ABI_IOS)
+#if defined(TARGET_OS_MACOS) || defined(TARGET_OS_MACOS_IOS)
   ASSERT(kExitLinkSlotFromEntryFp == -26);
-#elif defined(TARGET_ABI_EABI)
+#else
   ASSERT(kExitLinkSlotFromEntryFp == -27);
 #endif
   __ Push(R9);