vppinfra: clib_memcpy optimization

vppinfra: clib_memcpy optimization

This patch allows copiler to select which SIMD instructions
will be used for copying 16 and 32 byte block.

Immediate effect of this change will occur in avx2 variants of
graph node functions. So far 128 byte registers were used
even in code optimized for avx2 due to macro nature of
clib_memcpy. With this patch gcc should pick 256 byte registers
in such cases.

Change-Id: I3510ee9b3bf01f3f0a9184e1a3f8e1bd827f8eee
Signed-off-by: Damjan Marion <[email protected]>