Windows toolchain update

Hi Windows devs, The Windows GCC has been updated to 6.2.0 and binutils to 2.27. At some point please rebuild using these binaries. Do throw away your old toolchain cache before getting the new one: rm -rf ghc-tarballs && ./configure --enable-tarballs-autodownload The plan is to ship these versions with GHC 8.2, though binutils will likely Get another minor bump. Thanks, Tamar