Binutils: ARC: Fix build failures if makeinfo is missing

Binutils: ARC: Fix build failures if makeinfo is missing

Build failed when "makeinfo" was missing on the build host.
This was happening because "makeinfo" is required to build .info targets
and make exited with error. The issue hadn't appeared before as there was
prebuilt documentation in ARC binutils tarballs, so no attempts had been
made to build docs.

Missing "makeinfo" only stops us from building docs
("missing" script already throws a warning on that regard).
Let's continue to build other targets.

Now exit code of the script called "missing" is checked.
The value 127 means that "makeinfo" is not available on the build host.

So when such value occurs, 0 is returned to the top level makefile.
Documentation is not being built but further build of binutils continues.

Fixes http://autobuild.buildroot.net/results/55c/55cd09a559016f4f252f0e4c27313b9806135cf4//

Signed-off-by: Zakharov Vlad <vzakhar@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>