RFR: 8277924: Small tweaks to foreign function and memory API

[Maurizio Cimadamore]

Following integration of the second incubator of the foreign function and memory API [1], we detected few divergences between the contents of the jdk repo and the panama repo:

* the name of some of the `FunctionDescriptor` wither methods is different (e.g. `withAppendedLayoutArguments` vs. `appendLayoutArguments`), as it has been simplified and improved following a change that was not incorporated in [1].

* TestUpcall does not execute all the test combinations, because of an issue in the jtreg header (also fixed in the panama repo)

* Addressing some feedback, we would like to bring back alignment to JAVA_INT layout constants (and related constants). 

Javadoc: http://cr.openjdk.java.net/~mcimadamore/8277924/v1/javadoc/jdk/incubator/foreign/package-summary.html
Specdiff: http://cr.openjdk.java.net/~mcimadamore/8277924/v1/spec_diff/overview-summary.html

[1] - #5907

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Commit messages:
 - Further tweak to javadoc of layout constants
 - Wrong alignment constraints in ValueLayout constants javadoc
 - Tweak javadoc
 - Fix jtreg header on TestUpcall
 - Tweak API names for FunctionDescriptor methods
 - * Tweak value layout constants alignment to reflect VM alignment

Changes: https://git.openjdk.java.net/jdk/pull/6589/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6589&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277924
  Stats: 343 lines in 14 files changed: 206 ins; 5 del; 132 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6589.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6589/head:pull/6589

PR: https://git.openjdk.java.net/jdk/pull/6589