[arch-general] Adopt tar.xz for official repo packaging ?

[b4283]

Corrently Archlinux uses a tar+gzip combination and it works nicely.
But right now there's a stable lzma compression available using "tar Jcvf",
which could create much smaller packages and still fast decompressions.
(of course you need to have xz-utils)

think about all the bandwidth that could be saved !

my little experiment shows

	kernel26-2.6.30-5-x86_64.pkg.tar.gz
	coreutils-7.4-1-x86_64.pkg.tar.gz
tar.gz
	35338866 (100%)
	4853937 (100%)
tar.xz
	24892732 (70.4%)
	1843592 (38.2%)


i guess one of the major concerns is that tar & gzip is more common than xz.
but since GNU is also packing their coreutils in xz packs, i guess that 
a good time to switch?