RFR: 8241502: C2: Migrate x86_64.ad to MacroAssembler [v8]

[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.

John Tortugo has updated the pull request incrementally with one additional commit since the last revision:

  Fix cast of constant

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2420/files
  - new: https://git.openjdk.java.net/jdk/pull/2420/files/ef7a993c..4b0912ce

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2420&range=07
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2420&range=06-07

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