Fix build-mingw64-toolchain.sh

1) Default to use later gcc, gmp, mpc, and mpfr
2) HOST_BINPREFIX can be directory: eg.
   --binprefix=${HOME}/mydroid/prebuilts/tools/gcc-sdk/
3) --force-build now works

Change-Id: If9c3a9bea9dc14956ab1b535fbc1a05e75fa9db9
1 file changed