vppinfra: added performance test for clib_rwlock_t (test_rwlock.c)

vppinfra: added performance test for clib_rwlock_t (test_rwlock.c)

Spawns a uniform number of writer and reader threads across a number of
cores where each writer thread increments a global variable a specified
number of times, and the reader threads continually poll the global's
value until the writers complete.

Type: test

Change-Id: I979c3734c6d03139d0802bff1846875d226f6fbb
Signed-off-by: Jason Zhang <jason.zhang2@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Lijian Zhang <Lijian.Zhang@arm.com>