linphone: arc: Undefine __arc__ when building for ARC processors

This patch is similar to 821a6d63. Libexosip2 defines symbol __arc__, which
gets in contradiction with same symbol defined by the GCC for the Synopsys
DesignWare ARC cores. Previous patch undefined __arc__ for libosip2 and
libexosip2, however linphone includes headers from libexosip2, which causes
build failure unless same patch is applied to it.

This patch fixes
http://autobuild.buildroot.net/results/0d2e19d40d6d0df4048e6ebae5f77022f89ca7cb/



Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>