[foreign-memaccess+abi] 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.

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

Commit messages:
 - Update src/java.base/share/classes/jdk/internal/foreign/abi/aarch64/AArch64Architecture.java
 - Declare private constructors

Changes: https://git.openjdk.org/panama-foreign/pull/760/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=760&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298532
  Stats: 8 lines in 2 files changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/760.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/760/head:pull/760

PR: https://git.openjdk.org/panama-foreign/pull/760