RFR: 8241502: Migrate x86_64.ad to MacroAssembler

[John Tortugo]

Relates to: https://bugs.openjdk.java.net/browse/JDK-8241502
Tested on: Linux tier1, 2 and 3

Can you please take a look whether these changes are going in the direction expected or not? If it is, I'll continue working on the `JDK-8241502` but I'd like to split it in a few PRs since it's a lot of changes.

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Commit messages:
 - Merge branch 'jdk-8241502' of https://github.com/JohnTortugo/jdk into jdk-8241502
 - First part of Migrate x86_64 to MacroAssembler
 - First part of Migrate x86_64 to MacroAssembler
 - Merge pull request #1 from openjdk/master

Changes: https://git.openjdk.java.net/jdk/pull/2420/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2420&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8241502
  Stats: 174 lines in 1 file changed: 44 ins; 21 del; 109 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2420.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2420/head:pull/2420

PR: https://git.openjdk.java.net/jdk/pull/2420