RFR: 8308293: A linker should expose the layouts it supports

[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`.

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Commit messages:
 - Tweak javadoc
 - Initial push

Changes: https://git.openjdk.org/jdk/pull/14037/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14037&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308293
  Stats: 105 lines in 3 files changed: 74 ins; 5 del; 26 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