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

runtime/vm/constants_arm.h

Index: runtime/vm/constants_arm.h
diff --git a/runtime/vm/constants_arm.h b/runtime/vm/constants_arm.h
index c4213ad0451be3fed5374203ab35cb6ace34eb9b..89ac511f97cc7afe11ec925bce25cffaa9d3fcde 100644
--- a/runtime/vm/constants_arm.h
+++ b/runtime/vm/constants_arm.h
@@ -81,14 +81,12 @@ enum Register {
   kNoRegister = -1,  // Signals an illegal register.
 
 // Aliases.
-#if defined(TARGET_ABI_IOS)
+#if defined(TARGET_OS_MACOS) || defined(TARGET_OS_MACOS_IOS)
   FP = R7,
   NOTFP = R11,
-#elif defined(TARGET_ABI_EABI)
+#else
   FP = R11,
   NOTFP = R7,
-#else
-#error Unknown ABI
 #endif
   IP = R12,
   SP = R13,
@@ -305,17 +303,15 @@ const RegList kAllCpuRegistersList = 0xFFFF;
 // C++ ABI call registers.
 const RegList kAbiArgumentCpuRegs =
     (1 << R0) | (1 << R1) | (1 << R2) | (1 << R3);
-#if defined(TARGET_ABI_IOS)
+#if defined(TARGET_OS_MACOS) || defined(TARGET_OS_MACOS_IOS)
 const RegList kAbiPreservedCpuRegs =
     (1 << R4) | (1 << R5) | (1 << R6) | (1 << R8) | (1 << R10) | (1 << R11);
 const int kAbiPreservedCpuRegCount = 6;
-#elif defined(TARGET_ABI_EABI)
+#else
 const RegList kAbiPreservedCpuRegs = (1 << R4) | (1 << R5) | (1 << R6) |
                                      (1 << R7) | (1 << R8) | (1 << R9) |
                                      (1 << R10);
 const int kAbiPreservedCpuRegCount = 7;
-#else
-#error Unknown ABI
 #endif
 const QRegister kAbiFirstPreservedFpuReg = Q4;
 const QRegister kAbiLastPreservedFpuReg = Q7;
@@ -329,9 +325,9 @@ const RegList kDartAvailableCpuRegs =
 // Registers available to Dart that are not preserved by runtime calls.
 const RegList kDartVolatileCpuRegs =
     kDartAvailableCpuRegs & ~kAbiPreservedCpuRegs;
-#if defined(TARGET_ABI_IOS)
+#if defined(TARGET_OS_MACOS) || defined(TARGET_OS_MACOS_IOS)
 const int kDartVolatileCpuRegCount = 6;
-#elif defined(TARGET_ABI_EABI)
+#else
 const int kDartVolatileCpuRegCount = 5;
 #endif
 const QRegister kDartFirstVolatileFpuReg = Q0;