package/libbsd: not available for ARC

package/libbsd: not available for ARC

libbsd has explicit, hard-coded checks about the architectures it can
work on, and ARC is not one of those.

We did not notice so far, because we only recently added support for
glibc on ARC (and only for a single variant) in 0633eb58a291 (toolchain:
add glibc support for ARCv2).

Add an explicit exclusion on arc.

Fixes:
    http://autobuild.buildroot.org/results/603baa77e95620ad1416e0d1dc4202c334801efc
    http://autobuild.buildroot.org/results/8a2ee5431501615cb150233e6d7bc9e7c3c5c1eb
    http://autobuild.buildroot.org/results/ea52364f536485ff4e43e3bc37f2175eb6178c5a
    http://autobuild.buildroot.org/results/32581f7a79372b525e4ad21e029ff0ede743ba94

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Evgeniy Didin <Evgeniy.Didin@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>