eal: fix unused value warning in memcpy macro

eal: fix unused value warning in memcpy macro

GCC 4.5.1 from SUSE throws this error:
	lib/librte_pmd_enic/enic_main.c:862:2: error: value computed is not used

This change use statements in expressions C extension provided by gcc to avoid
'value computed is not used' warning/error when size is not known at compile
time.

Reported-by: Michael Qiu <michael.qiu@intel.com>
Signed-off-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Acked-by: Michael Qiu <michael.qiu@intel.com>
[Thomas: apply same fix to ppc_64]
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>