Summary
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Remove all of the sources related to the `arm64` port while retaining the 32-bit ARM port and the 64-bit `aarch64` port.

Motivation
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Removing this port will allow all contributors to focus their efforts on a single 64-bit ARM implementation, and eliminate the duplicate work required to maintain two ports.

Description
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Two 64-bit ARM ports exist in the JDK.  The main sources for these are in the `src/hotspot/cpu/arm` and `open/src/hotspot/cpu/aarch64` directories.  Although both ports produce `aarch64` implementations, for the sake of this JEP we shall refer to the former, which was contributed by Oracle, as `arm64`, and the latter as `aarch64`.

Here are the tasks that will be completed as part of this JEP:

 - Remove all `arm64`-specific sources and `#ifdef`s related to 64-bit versus 32-bit builds in `open/src/hotspot/cpu/arm`
 - Scan the remaining JDK sources for `#ifdef`s related to this port
 - Remove the build option for building this port.  Make the `aarch64` port the default build for the 64-bit ARM architecture.
 - Validate that the remaining 32-bit ARM port continues to build and run the conformance tests with no regressions that did not exist prior to the changes related to this JEP.