RFR: 8253639: Change os::attempt_reserve_memory_at parameter order

[Stefan Karlsson]

I propose that we change the parameter order from:
os::attempt_reserve_memory_at(size, addr, <extra params>)

to:
os::attempt_reserve_memory_at(addr, size, <extra params>)

This patch builds upon the changes in pull/357. There's currently no way to create an incremental review, but I want to
get this out for review now. So, unfortunately you have to ignore the first commit and look at the second commit.

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Commit messages:
 - 8253639: Change os::attempt_reserve_memory_at parameter order
 - 8253638: Cleanup os::reserve_memory and remove MAP_FIXED

Changes: https://git.openjdk.java.net/jdk/pull/359/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=359&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253639
  Stats: 164 lines in 18 files changed: 29 ins; 59 del; 76 mod
  Patch: https://git.openjdk.java.net/jdk/pull/359.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/359/head:pull/359

PR: https://git.openjdk.java.net/jdk/pull/359