eal/x86: select optimized memcpy at run-time

eal/x86: select optimized memcpy at run-time

This patch dynamically selects functions of memcpy at run-time based
on CPU flags that current machine supports. This patch uses function
pointers which are bind to the relative functions at constrctor time.
In addition, AVX512 instructions set would be compiled only if users
config it enabled and the compiler supports it.

Signed-off-by: Xiaoyun Li <xiaoyun.li@intel.com>