sanity: check for GNU tar specifically

We need the system tar to be GNU tar, as we reply on --xattrs.  Some
distributions may be using libarchive's tar binary, which is definitely
not as featureful, so check for this and abort early with a clear
message instead of later with mysterious errors.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>