buildroot

stress-ng: not available on ARC

Now that ARC supports glibc, stress-ng can be built. Unfortunately,
like NIOS2, it doesn't implement FE_INVALID and a bunch of other
<fenv.h> definitions, so let's prevent stress-ng from being built on
ARC.

Fixes:

  http://autobuild.buildroot.net/results/296b14584c200593f88af75cdda65c4ca03cd863/

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