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

Change-Id: I0e30cb938968cadd21968c3598ddce59f5f0ca0f
Reviewed-on: https://go-review.googlesource.com/46772
Reviewed-by: Ian Lance Taylor <iant@golang.org>
3 files changed