RFR: 8308293: A linker should expose the layouts it supports [v5]

[Maurizio Cimadamore]

> This patch adds an instance method on `Linker`, namely `Linker::canonicalLayouts` which returns all the layouts known by the linker as implementing some ABI type. For instance, if I call this on my machine (Linux/x64) I get this:
> 
> 
> jshell> import java.lang.foreign.*;
> 
> jshell> Linker.nativeLinker().canonicalLayouts()
> $2 ==> {char16_t=c16, int8_t=b8, long=j64, size_t=j64, bool=z8, int=i32, long long=j64, int64_t=j64, void*=a64, float=f32, char=b8, int16_t=s16, int32_t=i32, short=s16, double=d64}
> 
> 
> This can be useful to discover the ABI types supported by a linker implementation, as well as for, in the future, add support for more exotic (and platform-dependent) linker types, such as `long double` or `complex long`.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Drop "unsigned short"
  Beef up javadoc

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14037/files
  - new: https://git.openjdk.org/jdk/pull/14037/files/dbc8049a..708f987d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14037&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14037&range=03-04

  Stats: 35 lines in 5 files changed: 6 ins; 13 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/14037.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14037/head:pull/14037

PR: https://git.openjdk.org/jdk/pull/14037