On AIX64 pthread_t is of type unsigned int (4bytes),
using intptr_t instead when casting a pointer.

R=lpy@chromium.org, jochen@chromium.org, mbrandy@us.ibm.com

BUG=
LOG=N

Review-Url: https://codereview.chromium.org/1938833002
Cr-Commit-Position: refs/heads/master@{#36141}