[vm] Fix feature detection of DBC32 versus DBC64 in snapshots with code.

At a minimium, the image portion of the snapshot is sensitive to word size.

Change-Id: I1056a726c7b773f3ef65608f3244aaa01a366ec4
Reviewed-on: https://dart-review.googlesource.com/56023
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>
diff --git a/runtime/vm/dart.cc b/runtime/vm/dart.cc
index 09ce27c..e845697 100644
--- a/runtime/vm/dart.cc
+++ b/runtime/vm/dart.cc
@@ -700,15 +700,21 @@
 #elif defined(TARGET_ARCH_IA32)
     buffer.AddString(" ia32");
 #elif defined(TARGET_ARCH_X64)
-#if defined(_WIN64)
+#if defined(TARGET_OS_WINDOWS)
     buffer.AddString(" x64-win");
 #else
     buffer.AddString(" x64-sysv");
 #endif
 #elif defined(TARGET_ARCH_DBC)
-    buffer.AddString(" dbc");
-#elif defined(TARGET_ARCH_DBC64)
+#if defined(ARCH_IS_32_BIT)
+    buffer.AddString(" dbc32");
+#elif defined(ARCH_IS_64_BIT)
     buffer.AddString(" dbc64");
+#else
+#error What word size?
+#endif
+#else
+#error What architecture?
 #endif
   }