Ported from arm NEON and added vector_dmul_scalar.
Functions between 1.5 and 5 times faster than the C implementations
using Apple's clang-503.0.19 on A7.
NEON and VFP are currently mandatory for all ARMv8 profiles. Both are
handled as extensions as far as cpuflags are concerned. This is
consistent with handling x86_64 which always has SSE2, but still
handles it as an extension.
