[aix] os::release_memory may not release the full range

JDK-8255978 fixed a bug on Windows where it was not possible to release an address range consisting of multiple mappings, and added a regression gtest (os release_multi_mappings).

Ironically, that test now fails on AIX, where we use SystemV shared memory which has the same semantics as the Windows virtual memory API. See also JDK-8257040.

(Note that we may have a similar problem on Linux where we use System V shm for large pages with +UseSHM - but there, this type of memory is restricted to os::reserve_memory_special, and therefore the os.release_one_mapping_multi_commits_vm gtest does not fail, since it only tests the interplay of os::release_memory() with the standard os::reserve_memory()).