windows/py: Support 64bit mingw-w64 builds

- add mp_int_t/mp_uint_t typedefs in mpconfigport.h
- fix integer suffixes/formatting in mpconfig.h and mpz.h
- use MICROPY_NLR_SETJMP=1 in Makefile since the current nlrx64.S
  implementation causes segfaults in gc_free()
- update README