Disable test for 32-bit systems (e.g. ARM, i386)

Both i386 and 32-bit ARM do not define __uint128_t. On most systems, if
__uint128_t is defined, then so is the macro __SIZEOF_INT128__.

https://stackoverflow.com/questions/18531782/how-to-know-if-uint128-t-is-defined1

unsupported/test/cxx11_tensor_uint128.cpp

@@ -12,7 +12,7 @@
 #include <Eigen/CXX11/Tensor>
 
 
-#if EIGEN_COMP_MSVC
+#if EIGEN_COMP_MSVC || !defined(__SIZEOF_INT128__)
 #define EIGEN_NO_INT128
 #else
 typedef __uint128_t uint128_t;