Added support for building against Android API-9 SDK

[vharron]

Summary

Android's API-9 SDK is missing log2.

[vharron]

fixed comment

[vharron]

Should I submit this as is or create a

lib/Support/Math.h
lib/Support/Math.cpp

That just wraps log2 on most platforms and does the right thing on Android.

[vharron]

include/llvm/Support/Math.h

[vharron]

created an abstraction for log2

Added llvm::Log2 in Support/MathExtras.h

Hid Android problems inside of it

lib/Analysis/ConstantFolding.cpp

 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/Operator.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
 #include <cerrno>
 #include <cmath>
 
+#if defined(__ANDROID_NDK__)
+#include <android/api-level.h>
+
+#if __ANDROID_API__ < 18
+// log2 is missing from API < 18
+double log2(double d) { return (double)__builtin_log2l(d); }
+#else
+#include <math.h>
+#endif
+
+#endif // defined(__ANDROID_NDK__) && defined(__i386__)
+
 #ifdef HAVE_FENV_H
 #include <fenv.h>
 #endif
 
 using namespace llvm;
 
 //===----------------------------------------------------------------------===//
 // Constant Folding internal helper functions