package/librtas: only show comment on supported architectures

librtas is only available for PPC and PPC64, so only show the comment
about the (e)glibc dependency for those archs.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Sam Bobroff <sam.bobroff@au1.ibm.com>
Reviewed-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>