Add a basic TargetARM32 skeleton which knows nothing.

src/IceCompiler.cpp

Index: src/IceCompiler.cpp
diff --git a/src/IceCompiler.cpp b/src/IceCompiler.cpp
index 47f1ba407c2fa86adeec2209b551a0612f69223f..7c8b791969e63feafdac29b17421876e2640befc 100644
--- a/src/IceCompiler.cpp
+++ b/src/IceCompiler.cpp
@@ -47,9 +47,15 @@ struct {
 // Validates values of build attributes. Prints them to Stream if
 // Stream is non-null.
 void ValidateAndGenerateBuildAttributes(const ClFlags &Flags, Ostream *Stream) {
-  if (Stream)
+  if (Stream) {
+    // List the requested target.
     *Stream << Flags.getTargetArch() << "\n";
 
+// List the supported targets.
+#define SUBZERO_TARGET(TARGET) *Stream << "target_" #TARGET << "\n";
+#include "llvm/Config/SZTargets.def"
+  }
+
   for (size_t i = 0; i < llvm::array_lengthof(ConditionalBuildAttributes);
        ++i) {
     switch (ConditionalBuildAttributes[i].FlagValue) {