eal: fix clang build with intrinsics forced

eal: fix clang build with intrinsics forced

This fixes x86_64-native-linuxapp-clang build with
CONFIG_RTE_FORCE_INTRINSICS=y:

    include/generic/rte_atomic.h:218:9: error:
        implicit declaration of function '__atomic_exchange_2'
        is invalid in C99 [-Werror,-Wimplicit-function-declaration]

    include/generic/rte_atomic.h:501:9: error:
        implicit declaration of function '__atomic_exchange_4'
        is invalid in C99 [-Werror,-Wimplicit-function-declaration]

    include/generic/rte_atomic.h:783:9: error:
        implicit declaration of function '__atomic_exchange_8'
        is invalid in C99 [-Werror,-Wimplicit-function-declaration]

We didn't caught this issue previously on other platforms because
CONFIG_RTE_FORCE_INTRINSICS enabled by default only for armv8.

Fixes: 7bdccb93078e ("eal: fix ARM build with clang")
Cc: stable@dpdk.org
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>