Fix POWER8 and POWER9 build

Fix POWER8 and POWER9 build

Tested with GCC 6.4.0 and 8.1.0 in debug and release modes,
with and without SIMD.

xlc is still not supported, e.g. because 16.1 cannot handle an
in-class member initializer list for a C-style array in the same
struct as a member initialized in a constructor initializer
list. Tested for this in the C++11 feature support tests, and we now
issue a descriptive error in this case.

Removed -mdirect-move from POWER8 flags because gcc warns about its
use.

Fixes #2748

Change-Id: I0dc68c12701f041b702063692703cc2e15e96d8e