RFR: 8253638: Cleanup os::reserve_memory and remove MAP_FIXED

[Stefan Karlsson]

There's a code path through os::reserve_memory that uses MAP_FIXED. This path is used if a requested address is given.
It's very dangerous to use MAP_FIXED, since it causes pre-existing mappings to be replaced. Alternative, move safe,
functions exists; named os_attempt_reserve_memory_at.

It's not only dangerous to use that API, the Mac AARCH64 port also can't use it in conjunction with its MAP_JIT.

I started to split os::reserve_memory into two functions; one that doesn't care about were the memory is mapped
(os::reserve_memory), and another that uses the dangerous MAP_FIXED flag (os::reserve_memory_at). However, I noticed
that it's only windows code that actually use os::reserve_memory with a requested address. All other usages have been
cleaned out and replaced with os::attempt_reserve_memory. And on windows os::attempt_reserve_memory uses
os::reserve_memory with comments that it's safe because it only "attempts" to place the mapping.

So, this patch:
1) Removes the forcefully reserving of memory and its MAP_FIXED usages!
2) Changes the windows code to call the attempt_reserve_memory_at version, that it actually did anyway.
3) Flips the call order between pd_reserve_memory and pd_attempt_reserve_memory_at, on windows.
4) There's also some unification and split done to handle MEMFLAGS and fds. This part needs followup cleanups (IMHO).
5) The AIX implementation already refused to use MAP_FIXED, and now it doesn't need special code to handle that.
6) Small cleanups around anon_mmap and anon_mmap_aligned

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Commit messages:
 - 8253638: Cleanup os::reserve_memory and remove MAP_FIXED

Changes: https://git.openjdk.java.net/jdk/pull/357/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=357&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253638
  Stats: 143 lines in 17 files changed: 29 ins; 59 del; 55 mod
  Patch: https://git.openjdk.java.net/jdk/pull/357.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/357/head:pull/357

PR: https://git.openjdk.java.net/jdk/pull/357