runtime: adapt memory management to AIX mmap

runtime: adapt memory management to AIX mmap

    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