RFR: 8277924: Small tweaks to foreign function and memory API [v3]

[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

Maurizio Cimadamore has updated the pull request incrementally with two additional commits since the last revision:

 - Drop changes to alignment of layout constants
 - Simplify logic to access package-private method in ValueLayout

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6589/files
  - new: https://git.openjdk.java.net/jdk/pull/6589/files/54b89f30..e2dfb83b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6589&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6589&range=01-02

  Stats: 255 lines in 6 files changed: 0 ins; 159 del; 96 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