ARM64 support (ABC-436)

[thomas-tu]

Purpose of this PR

Ticket/Jira #: https://jira.unity3d.com/browse/ABC-436

Changes:

• Depends on ISPC 1.21.0 (was 1.14.1).
• Depends on Alembic 1.8.5 (was 1.7.16).
• Small change on internal API (was crashing on ARM64 platform).
• Updated cmake files to generate ARM64 .dll. There's now a new .meta file in the package Plugins/ folder with specific import settings for the new .dll.

Note: at the moment, ARM64 .dll is manually built until we can make it automatic in the build pipeline.

Testing

Functional Testing status:

• Manually tested on ARM64 platform.
• Usually CI tests.

Performance Testing status:

Overall Product Risks

Complexity: Low

Halo Effect: Medium

Documentation & UX Writing

User facing text to review		Details
User interface	N
Public API docs	N
Changelog	Y

Documentation halo effect		Jira link
User manual	N
Other docs	N

Additional information

Note to reviewers:

Reminder:

• [x] Add entry in CHANGELOG.md