Fix GCC 4.8 mingw build

Export CXX_FOR_BUILD otherwise the default g++ can have different
bitness than libiberty.a which is always built in 32-bit for now.
Didn't happen prior to GCC 4.8 because GCC was in C only.  GCC from
4.8 now uses C++ as its implementation language

Change-Id: I318a2e469e1f1ce5cea71654ca5387d26cee5b73