vppinfra: Expose function setting __os_thread_index

vppinfra: Expose function setting __os_thread_index

Type: feature

This is needed when creating pthreads in client applications,
they need a way to set __os_thread_index per thread
that does not conflict with the binary API thread index.

If __os_thread_index is left to 0 in two client pthreads and
they call vl_msg_api_alloc and vec_resize at the same time
it can fail due to them sharing (and push/poping) the same
clib_per_cpu_mheaps slot.

Change-Id: I85d4248a39b641a4d3ad5a1c1bd6e0db5875fab6
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>