RFR: JDK-8263884: Clean up os::is_allocatable() across Posix platforms [v2]

[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.

Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:

  remove unnecessary cast

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3092/files
  - new: https://git.openjdk.java.net/jdk/pull/3092/files/ffeb9bd6..287abd7e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3092&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3092&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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