eal: add generic thread-local-storage functions

eal: add generic thread-local-storage functions

Add support for TLS functionality in EAL.

The following functions are added:
rte_thread_tls_key_create - create a TLS data key.
rte_thread_tls_key_delete - delete a TLS data key.
rte_thread_tls_value_set - set value bound to the TLS key
rte_thread_tls_value_get - get value bound to the TLS key

TLS key is defined by the new type rte_tls_key.

The API allocates the thread local storage (TLS) key.
Any thread of the process can subsequently use this key
to store and retrieve values that are local to the thread.

Those functions are added in addition to TLS capability
in rte_per_lcore.h to allow abstraction of the pthread
layer for all operating systems.

Windows implementation is under librte_eal/windows and
implemented using WIN32 API for Windows only.

Unix implementation is under librte_eal/unix and
implemented using pthread for UNIX compilation.

Signed-off-by: Tal Shnaiderman <talshn@nvidia.com>
Acked-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>