[C++] Fix and enable UBSan for unaligned accesses.

[asfimport]

Currently unaligned access configuration in UBSan has been turned off. We should introduce a method that safely loads unaligned data, and use it to fix UBSan errors. Then turn them on for UBSan.

Reporter: Micah Kornfield / @emkornfield
Assignee: Micah Kornfield / @emkornfield

PRs and other links:

• GitHub Pull Request #4757
• GitHub Pull Request #4787

_{Note: This issue was originally created as ARROW-5380. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 4757
#4757