In Ubuntu Bionic for armhf, clang is compiled for armv8l rather than
armv7l (as it was and still is for gcc) and so it uses armv8 by default.

This breaks compilation of tests using smp_mb():

> error: instruction requires: data-barriers

The fix is to add "-march=armv7-a" to CFLAGS which we already do,
except not for the tests.
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>