RFR: JDK-8304539: Cleanup utilities/{count_leading_zeros,count_trailing_zeros,population_count}.hpp

[Justin King]

As the title says, cleanup the mentioned headers. This is similar to `byteswap.hpp` and removes the extraneous `#ifdef` for XLC since it is really just Clang now.

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Commit messages:
 - Merge remote-tracking branch 'upstream/master' into popcount
 - Update copyright
 - Cleanup based on byteswap.hpp
 - Refactor count_leading_zeros, count_trailing_zeros, and population_count

Changes: https://git.openjdk.org/jdk/pull/13103/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13103&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304539
  Stats: 413 lines in 3 files changed: 184 ins; 155 del; 74 mod
  Patch: https://git.openjdk.org/jdk/pull/13103.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13103/head:pull/13103

PR: https://git.openjdk.org/jdk/pull/13103