MFH: r499211

devel/icu: update to 64.2

Changes: https://github.com/unicode-org/icu/releases/tag/release-64-2
Changes: http://site.icu-project.org/download/64
ABI: https://abi-laboratory.pro/tracker/timeline/icu4c/
PR: 236325
Approved by: ports-secteam blanket (required by Firefox 60.7esr)