enable GCC large address aware linker flag (Windows only)

- this flag allows bitcoin-qt.exe / bitcoind.exe (32-bit application) to
  handle addresses larger than 2GB (up to 3GB on x86 Windows and up to
  4GB on x64 Windows)
Merge pull request #2167 from Diapolo/Qt_laa_flag

enable GCC large address aware linker flag (Windows only)