Fix Mac OS compilation.

src/IceInstX8664.def

@@ -214,11 +214,9 @@
 #if defined(_WIN32) && defined(SUBZERO_USE_MICROSOFT_ABI)  // Microsoft x86-64 ABI
 #define REGX8664_BYTEREG_TABLE REGX8664_BYTEREG_TABLE2(0, 1)
 #define REGX8664_GPR_TABLE REGX8664_GPR_TABLE2(0, 1)
-#elif defined(__unix__)  // System V AMD64 ABI
+#else  // System V AMD64 ABI
 #define REGX8664_BYTEREG_TABLE REGX8664_BYTEREG_TABLE2(1, 0)
 #define REGX8664_GPR_TABLE REGX8664_GPR_TABLE2(1, 0)
-#else
-#error "Unsupported platform"
 #endif
 
 // Note: It would be more appropriate to list the xmm register aliases as

src/IceTargetLoweringX8664Traits.h

@@ -730,7 +730,7 @@ public:
     assert(Ty == IceType_i64 || Ty == IceType_i32);
     return getGprForType(Ty, GprForArgNum[ArgNum]);
   }
-#elif defined(__unix__)
+#else
   // System V x86-64 calling convention:
   //
   // * The first eight arguments of vector/fp type, regardless of their
@@ -762,8 +762,6 @@ public:
     assert(Ty == IceType_i64 || Ty == IceType_i32);
     return getGprForType(Ty, GprForArgNum[ArgNum]);
   }
-#else
-#error "Unsupported platform"
 #endif
 
   /// Whether scalar floating point arguments are passed in XMM registers