Libarchive can use either libxml2 or libexpat for xar support, but the
LIBXML2 option currently makes libarchive link to both libxml2 and
libbsdxml (base system private copy of libexpat). Replace this with a
simple depdendency on libexpat.

Also remove PATCH_STRIP because there are no patches, replace MAKE with
MAKE_CMD and add INSTALL_TARGET=install-strip.

PR: 201948
Approved by: glewis (maintainer)