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

[Per Minborg]

On Mon, 12 Dec 2022 08:37:02 GMT, Per Minborg <pminborg at openjdk.org> wrote:

> 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.

Closed in favour of https://github.com/openjdk/panama-foreign/pull/760

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