scripts/functions: add xz support

Add support for the new XZ-compressed tarballs. At least glibc
uses that new format.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

reporter_name="Yann E. MORIN"

reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

reporter_comment="EXPERIMENTAL MIPS-64 n64 toolchain."