[Doc] Update mingw build instructions for 64-bit
[Doc] Update mingw build instructions for 64-bit

Previous Windows mingw build instructions only described how to build
bitcoin for 32-bit.  The instructions have now been updated to also
describe how to build for 64-bit.

Bitcoin-com/BUcash,marlengit/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,krzysztofwos/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,Bitcoin-com/BUcash,BTCfork/hardfork_prototype_1_mvf-bu,marlengit/BitcoinUnlimited,Bitcoin-com/BUcash,marcusdiaz/BitcoinUnlimited,cddjr/BitcoinUnlimited,cddjr/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,marlengit/BitcoinUnlimited,cddjr/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,BTCfork/hardfork_prototype_1_mvf-bu,BTCfork/hardfork_prototype_1_mvf-bu,marcusdiaz/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,marlengit/BitcoinUnlimited,marlengit/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,marcusdiaz/BitcoinUnlimited,cddjr/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Bitcoin-com/BUcash,Justaphf/BitcoinUnlimited,marlengit/BitcoinUnlimited,cddjr/BitcoinUnlimited,Justaphf/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,marcusdiaz/BitcoinUnlimited,cddjr/BitcoinUnlimited,Bitcoin-com/BUcash,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,krzysztofwos/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,Bitcoin-com/BUcash,Justaphf/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,BTCfork/hardfork_prototype_1_mvf-bu