Make AVX2 check more robust

Apparently the GNU assembler version 2.20 supported some
parts of AVX2 (in particular the FMA floating-point ops),
which caused our cmake test to pass. This patch changes
it to test the integer addition instead, which isn't
supported in as-2.20 in my tests.

Change-Id: I79e516639fc78d1590d306f486eb027962388543