RFR: 8298532: Declare private constructors for FFM internal Architecture implementations

[Per Minborg]

This PR proposes declaring `AArch64Architecture` and `X86_64Architecture` `final` and creating a `private` constructor so that this redundant byte code can be eliminated:


 public jdk.internal.foreign.abi.x64.X86_64Architecture(); 
    Code: 
       0: aload_0 
       1: invokespecial #1 // Method java/lang/Object."<init>":()V 
       4: return 

 public jdk.internal.foreign.abi.aarch64.AArch64Architectur(); 
    Code: 
       0: aload_0 
       1: invokespecial #1 // Method java/lang/Object."<init>":()V 
       4: return 


and other potential optimizations can be unlocked.

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Commit messages:
 - Declare constructors private and classes final

Changes: https://git.openjdk.org/jdk/pull/11629/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11629&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298532
  Stats: 6 lines in 2 files changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11629.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11629/head:pull/11629

PR: https://git.openjdk.org/jdk/pull/11629