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