Pregeneration of NASM build artifacts

[justsmth]

Issues:

Addresses #364

Description of changes:

• Provide prebuilt NASM artifacts for build environments that lack a NASM assembler.

Call-outs:

• Usage of NASM artifacts is an "opt-in". It is determined as follows (subject to further discussion):
  • AWS_LC_SYS_PREBUILT_NASM=1 - Prebuilt artifacts used only if NASM was not found.
  • AWS_LC_SYS_PREBUILT_NASM=0 - NASM is required.
  • Unset - NASM is required.

Testing:

• Added the "prebuilt NASM verification" step that disassembles newly built NASM objects. It compares their disassembly to the disassembly of the NASM objects already in the repo. The CI job fails if they differ.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.48%. Comparing base (c358484) to head (144c689). Report is 64 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #491 +/- ## ========================================== - Coverage 95.80% 92.48% -3.32% ========================================== Files 61 67 +6 Lines 8143 9235 +1092 Branches 0 9235 +9235 ========================================== + Hits 7801 8541 +740 - Misses 342 422 +80 - Partials 0 272 +272 ```

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[djc]

Have you considered using a Cargo feature instead of an environment variable to make this work? That seems like a solution with better ergonomics for most downstream Rust applications.

[justsmth]

Have you considered using a Cargo feature instead of an environment variable to make this work? That seems like a solution with better ergonomics for most downstream Rust applications.

We considered using a feature for this. The concern is that we want the decision of whether it's acceptable to use the pre-built binaries to be made by the end consumer within their build environment. Because features are only "additive" and features can be enabled by any crate that's part of a build, it seems like the wrong mechanism.

As a feature, the consumer would essentially be required to "delegate" the decision of whether to use NASM pre-built binaries to any/every crate in their dependency tree. One possible compromise might be to expose a feature that specifies what to default to when AWS_LC_SYS_PREBUILT_NASM is not set in the environment -- but this also seems to violate the nature/purpose of "features" as the environment will be "subtracting" a "feature" that a dependent crate indicated that it needed.

We considered both "environment variables" and "features" as mechanisms for enabling this. Is there another option that would also enable intermediate crates (i.e., those part of the dependency tree between us and the end consumer) to influence what the default choice is?

[djc]

We considered both "environment variables" and "features" as mechanisms for enabling this. Is there another option that would also enable intermediate crates (i.e., those part of the dependency tree between us and the end consumer) to influence what the default choice is?

No, I don't think so. Your rationale makes sense to me, unfortunately Cargo doesn't support anything like this right now (see https://internals.rust-lang.org/t/pre-rfc-mutually-excusive-global-features/19618 for some discussion -- and if you think there is appetite for sponsoring work on this at AWS, I'd be interested in discussing that!).

As a downstream application builder I would still prefer being able to control this via features, maybe just with documentation guidance that libraries should not touch this and potentially keeping a way to override it via the environment variable?

[justsmth]

@djc -- Thanks for your feedback!

We are still discussing this issue internally. We may follow up in a subsequent PR to adjust the build environment conditions that determine whether the pre-built NASM objects are allowed to be used.