On AIX:
    * mmap does not allow to map an already mapped range,
    * mmap range start at 0x30000000 for 32 bits processes,
    * mmap range start at 0x70000000_00000000 for 64 bits processes
    
    This is adapted from change 37845.
    
    Issue golang/go#19200
    
    Reviewed-on: https://go-review.googlesource.com/46772

From-SVN: r249713