icu: add clang64

This is something that I've already gotten building, but was not needed for bootstrapping.

mingw-w64-icu/PKGBUILD

@@ -8,7 +8,7 @@ pkgname=("${MINGW_PACKAGE_PREFIX}-${_realname}" "${MINGW_PACKAGE_PREFIX}-${_real
 pkgver=68.2
 pkgrel=3
 arch=('any')
-mingw_arch=('mingw32' 'mingw64' 'ucrt64')
+mingw_arch=('mingw32' 'mingw64' 'ucrt64' 'clang64')
 pkgdesc="International Components for Unicode library (mingw-w64)"
 url="http://site.icu-project.org/home/"
 license=('custom:icu')