AIX doesn't allow to mmap an address range which is already mmap.
    Therefore, once the region has been allocated, it must munmap before
    being able to play with it.
    The corresponding Go Toolchain patch is CL 174059.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/174138

From-SVN: r270615