libarchive: bump to version 2.8.4

The libxml2 dependency is new in this version, and does not behave nicely with
cross compile. The configure script looks for the host xml2-config. Add
'--disable-xml2' as a workaround for now.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>