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

runtime/vm/disassembler_arm.cc

Index: runtime/vm/disassembler_arm.cc
diff --git a/runtime/vm/disassembler_arm.cc b/runtime/vm/disassembler_arm.cc
index 8c6d5fdee6afcec7d9768e6bce267cc93364a250..162d39f0aa108253c579fbcfc506b6f2b52690dd 100644
--- a/runtime/vm/disassembler_arm.cc
+++ b/runtime/vm/disassembler_arm.cc
@@ -112,14 +112,12 @@ void ARMDecoder::PrintCondition(Instr* instr) {
 // formatting, except for register alias pp (r5).
 // See for example the command "objdump -d <binary file>".
 static const char* reg_names[kNumberOfCpuRegisters] = {
-#if defined(TARGET_ABI_IOS)
+#if defined(TARGET_OS_MACOS) || defined(TARGET_OS_MACOS_IOS)
     "r0", "r1", "r2",  "r3",  "r4", "pp", "r6", "fp",
     "r8", "r9", "thr", "r11", "ip", "sp", "lr", "pc",
-#elif defined(TARGET_ABI_EABI)
+#else
     "r0", "r1", "r2",  "r3", "r4", "pp", "r6", "r7",
     "r8", "r9", "thr", "fp", "ip", "sp", "lr", "pc",
-#else
-#error Unknown ABI
 #endif
 };