libbsd: disable for xtensa and blackfin

libbsd: disable for xtensa and blackfin

Commit d879be30495 (package/libbsd: enable for non-glibc toolchains)
implicitly enabled libbsd for the uClibc only architectures xtensa and
blackfin. But libbsd does not support these architectures, since its
local-elf.h header lacks the required target specific definitions.

Fixes:
http://autobuild.buildroot.net/results/ff1/ff1a52a53bed43cda9e235081a61b352e6e07aa1/
http://autobuild.buildroot.net/results/b13/b13c2f248f277c72b5d266208f991f34474cea14/
http://autobuild.buildroot.net/results/182/182356701c2110a014c9d6c8d9f18a6efeb26e70/

Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>