docs: dev-tools: kmemleak: Update list of architectures

* Don't list powerpc twice (once as ppc)
* Drop tile, which has been removed from the source tree
* Mention arm64, nds32, arc, and xtensa

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20200303194215.23756-1-j.neuschaefer@gmx.net


Signed-off-by: Jonathan Corbet <corbet@lwn.net>