eal: fix ARM build with clang

eal: fix ARM build with clang

Use __atomic_exchange_n instead of __atomic_exchange_(2/4/8).

The error was:
	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

Fixes: ff2863570fcc ("eal: introduce atomic exchange operation")

Signed-off-by: Pavan Nikhilesh <pbhagavatula@caviumnetworks.com>