Don't assume pthread_t is uint_t for portability

Don't assume pthread_t is uint_t for portability

POSIX doesn't define pthread_t as uint_t. It could be a pointer.
This code causes below compile error on a platform using pointer
for pthread_t.

--
kernel.c:815:25: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
    (void) printf("%u ", (uint_t)pthread_self());

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #8558