eal: fix build of external app with clang on armv8

eal: fix build of external app with clang on armv8

In case DPDK built using GCC, RTE_TOOLCHAIN_CLANG is not defined.
But 'rte_atomic.h' is a generic header that included to the
external apps like OVS while building with DPDK. As a result,
clang build of OVS fails on armv8 if DPDK built using gcc:

    include/generic/rte_atomic.h:215:9: error:
            implicit declaration of function '__atomic_exchange_2'
            is invalid in C99
    include/generic/rte_atomic.h:494:9: error:
            implicit declaration of function '__atomic_exchange_4'
            is invalid in C99
    include/generic/rte_atomic.h:772:9: error:
            implicit declaration of function '__atomic_exchange_8'
            is invalid in C99

We need to check for current compiler, not the compiler used for
DPDK build.

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