eal: fix multiple definition of per lcore thread id

eal: fix multiple definition of per lcore thread id

Because of the inline accessor + static declaration in rte_gettid(),
we end up with multiple symbols for RTE_PER_LCORE(_thread_id).
Each compilation unit will pay a cost when accessing this information
for the first time.

$ nm build/app/dpdk-testpmd | grep per_lcore__thread_id
0000000000000054 d per_lcore__thread_id.5037
0000000000000040 d per_lcore__thread_id.5103
0000000000000048 d per_lcore__thread_id.5259
000000000000004c d per_lcore__thread_id.5259
0000000000000044 d per_lcore__thread_id.5933
0000000000000058 d per_lcore__thread_id.6261
0000000000000050 d per_lcore__thread_id.7378
000000000000005c d per_lcore__thread_id.7496
000000000000000c d per_lcore__thread_id.8016
0000000000000010 d per_lcore__thread_id.8431

Make it global as part of the DPDK_21 stable ABI.

Fixes: ef76436c6834 ("eal: get unique thread id")

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Ray Kinsella <mdr@ashroe.eu>
Reviewed-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>