bench: refactor to use dynamic memory allocation in blas/base/daxpy

[farazghani]

Resolves a part of #8643.

Description

What is the purpose of this pull request?

This pull request:

• {{This update follows the RFC guidance for replacing patterns such as:

double x[len];

with:

double *x;
x = (double *) malloc( len * sizeof( double ) );
...
free( x );}}

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• #{{TODO: add related issue number}}

Questions

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No.

Other

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No.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

AI Assistance

When authoring the changes proposed in this PR, did you use any kind of AI assistance?

• Yes
• No

If you answered "yes" above, how did you use AI assistance?

• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding

Disclosure

If you answered "yes" to using AI assistance, please provide a short disclosure indicating how you used AI assistance. This helps reviewers determine how much scrutiny to apply when reviewing your contribution. Example disclosures: "This PR was written primarily by Claude Code." or "I consulted ChatGPT to understand the codebase, but the proposed changes were fully authored manually by myself.".

{{TODO: add disclosure if applicable}}

---

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Coverage Report

Package	Statements	Branches	Functions	Lines
blas/base/daxpy	$\color{green}420/420$ $color{green}+100.00%$	$\color{green}30/30$ $color{green}+100.00%$	$\color{green}4/4$ $color{green}+100.00%$	$\color{green}420/420$ $color{green}+100.00%$

The above coverage report was generated for the changes in this PR.