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

py/mpz.h

@@ -73,7 +73,11 @@ typedef int8_t mpz_dbl_dig_signed_t;
 #endif
 
 #ifdef _WIN64
-  #define MPZ_LONG_1 1i64
+  #ifdef __MINGW32__
+    #define MPZ_LONG_1 1LL
+  #else
+    #define MPZ_LONG_1 1i64
+  #endif
 #else
   #define MPZ_LONG_1 1L
 #endif