[C++] unaligned access in compute/exec/ cc files

[asfimport]

See test-ubuntu-18.04-r-sanitizer log:

https://dev.azure.com/ursacomputing/crossbow/_build/results?buildId=6217&view=logs&j=0da5d1d9-276d-5173-c4c4-9d4d4ed14fdb&t=d9b15392-e4ce-5e4c-0c8c-b69645229181&l=842

PRs and other links:

• GitHub Pull Request #10489

_{Note: This issue was originally created as ARROW-13003. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 10489
#10489