I'm making this a bug instead of a feature request since uname -m is broken with regard to properly identifying a 64 bit machine. This causes issues when trying to guess the triplet for the build system where we want it to guess x86_64 for the CPU and the CPU supports 64 bit operations. We have clues in the environment that we might want to use.
Since MSYS is currently a 32bit binary PROCESSOR_ARCHITEW6432 is set to the previous value of PROCESSOR_ARCHITECTURE which is changed to x86 on my i7 CPU. I will issue a change to msys.bat that will check for these variables before setting MSYSTEM and if PROCESSOR_ARCHITECTURE or PROCESSOR_ARCHITEW6432 is AMD64 then MSYSTEM will have a value of MINGW64 else it will have a value of MINGW32. There maybe other values we want to consider but they may be too minimal to worry about.
While I've assigned this to Cesar I have every intention to provide a patch for review. However, if Cesar has time feel free to do the work.