RFR: 8308293: A linker should expose the layouts it supports [v6]

[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 with a new target base due to a merge or a rebase. The pull request now contains eight commits:

 - Merge branch 'master' into linker_types
 - Drop "unsigned short"
   Beef up javadoc
 - Address further review comments
 - Address review comments
 - More javadoc tweaks
 - Tweak javadoc
 - Tweak javadoc
   Add char type
 - Initial push

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Changes: https://git.openjdk.org/jdk/pull/14037/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14037&range=05
  Stats: 177 lines in 6 files changed: 121 ins; 16 del; 40 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