Make sure msys2 builds non emulating binaries
I came across this in my own project.
MSys2 doesn't enter #ifdef blocks which test for _WIN32. Instead, it pretends to be Linux and has a set of headers which emulate things like mmap.
These emulation functions are really really slow -- in my personal tests about 10x slower than calling the WINAPI -- so this ticket is just a reminder (it may already be done) for someone to check that MSys2 builds things non-emulated in GHC.