pthread.h: Convert pthread_t to unsigned long

pthread.h: Convert pthread_t to unsigned long

The POSIX standard outlines that pthread_t is an opaque data type and no
assumption should be made about it. However, we observed that libraries and
applications assume frequently that pthread_t is a pointer or other numerical
data type and use it to identify the corresponding threads.

pthread-embedded makes the strange decision to define pthread_t as
structure, a decision that, although compliant with POSIX standards,
makes the porting process a bit challenging. Therefore we introduce a
conversion function which simply returns the first encapsulated
unsigned long value in the structure which actually points to the
underlying thread object.

Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Felipe Huici <felipe.huici@neclab.eu>