RFR: JDK-8263884: Clean up os::is_allocatable() across Posix platforms

[Thomas Stuefe]

There exists a function `os::is_allocatable()` which is really only used on 32bit platforms if AggressiveHeap is set. That function tests if a given memory size can be mapped. A variant of this function exists for every os_cpu variant. They don't really differ much. Some of the 64bit only variants (eg ppc) never bothered to implement this.

I am not sure how much worth this function has, but it could at least be unified across POSIX platforms, and it should not use os::reserve_memory but plain raw mmap. There is no need for it to go through our whole reservation layer (eg NMT) if it just immediately releases the memory again.

I also would like NMT to be kept out of this since I work on NMT late stage initialization (JDK-8256844) and would like to avoid calling any of our reservation APIs before NMT initialization ran.

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Commit messages:
 - start

Changes: https://git.openjdk.java.net/jdk/pull/3092/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3092&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263884
  Stats: 116 lines in 13 files changed: 20 ins; 96 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3092.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3092/head:pull/3092

PR: https://git.openjdk.java.net/jdk/pull/3092