add minimal support for aarch64

[JakeHillion]

Summary: OI is currently completely incompatible with anything except x86_64. Changing that generally is a big effort, but oilgen should work on other architectures. Begin adding some support for aarch64 architecture.

This change sets up a file structure for architecture support. It pulls the 2 functions needed to make Descs.{h,cpp} architecture agnostic into architecture specific files for x86_64 and aarch64. This enables oilgen (the binary) to build. At this stage oilgen is unable to generate working code for aarch64, but at least this is a step in the right direction.

Differential Revision: D61661524

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This pull request was exported from Phabricator. Differential Revision: D61661524

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Codecov Report

Attention: Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.

Project coverage is 60.79%. Comparing base (8831269) to head (c70d6b2).

Files	Patch %	Lines
oi/Descs.h	0.00%	2 Missing :warning:
oi/arch/x86_64.cpp	60.00%	2 Missing :warning:
oi/Descs.cpp	66.66%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #507 +/- ## ========================================== - Coverage 60.80% 60.79% -0.01% ========================================== Files 126 127 +1 Lines 12493 12498 +5 Branches 2014 2014 ========================================== + Hits 7596 7598 +2 - Misses 3940 3943 +3 Partials 957 957 ```

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This pull request was exported from Phabricator. Differential Revision: D61661524

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This pull request was exported from Phabricator. Differential Revision: D61661524