mingw: Fix number of exponent digits in floating point formatting

By default mingw outputs 3 digits instead of the standard 2 so all float
tests using printf fail. Using setenv at the start of the program fixes this.
To accomodate calling platform specific initialization a
MICROPY_MAIN_INIT_FUNC macro is used which is called in mp_init()
Merge branch 'mingw-float-printf' of github.com:stinos/micropython into stinos-mingw-float-printf
windows: Argument to () function should be (void).
Merge branch 'stinos-mingw-float-printf'