[Packaging] Update linux packaging tasks to align with the LLVM 7 migration

[asfimport]

See #3499

Cosmic, Bionic, Xenial and Stretch tasks are failing:

• https://github.com/kszucs/crossbow/branches/all?query=ubuntu

• https://github.com/kszucs/crossbow/branches/all?query=debian
  respectively.

  We can pin clang-7 on Cosmic and Bionic, but I'm not sure what should be the strategy for Xenial and Stretch. @kou Do We need to make exceptions like with Trusty?

Reporter: Krisztian Szucs / @kszucs
Assignee: Kouhei Sutou / @kou

PRs and other links:

• GitHub Pull Request #3618

_{Note: This issue was originally created as ARROW-4527. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
I want Gandiva package at least Debian stretch.
We can use https://apt.llvm.org/ for clang-7 on Debian stretch and Ubuntu Xenial.

I take this issue.

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 3618
#3618