[C++] Consolidate utf8 benchmark

[asfimport]

I find some trivial (and obviously irrelevant) changes to UTF8 validation code may cause big variances to benchmark result.
UTF8 validation functions are inlined and called directly in benchmark. The compiler may try to optimize them together with the benchmark loop.
Un-inline the benchmark-ed functions makes the result predictable and explainable.

Reporter: Yibo Cai / @cyb70289
Assignee: Yibo Cai / @cyb70289

PRs and other links:

• GitHub Pull Request #11376

_{Note: This issue was originally created as ARROW-14269. Please see the migration documentation for further details.}

[asfimport]

Yibo Cai / @cyb70289:
Issue resolved by pull request 11376
#11376