[llvm-strings] Fix build linking against incorrect libiconv (#241766)
After https://github.com/llvm/llvm-project/pull/221794 the macOS build
breaks
if you have both the system and a GNU libiconv (e.g. from macports) on
the
include path. `#include <iconv.h>` will include the headers from the
package
manager but the link step will link the system library.
Using find_package + Iconv::Iconv passes the explicit library path to
the
linker which fixes the mismatch.
The ja_JP.SJIS test line is skipped on Darwin because the GNU + macOS
libiconvs
diverge on it.