libosip2: arc: Undefine __arc__ when building for ARC processors

libosip2 appears to have support for the "ARC standard" (Advanced
RISC Computing), which is unrelated to Synopsys DesignWare ARC
processors. The check for this involves verifying the existence of
the "__arc__" macro. Unfortunately, this macro is also present
as a predefined built-in for the ARC processors, resulting into
build failures.

[Peter: add short comment in .mk files as well]
Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>