htl: Initialize ___pthread_self early

When using jemalloc, malloc() needs to use TSD, while libpthread
initialization needs malloc(). Having ___pthread_self set early to some
static storage allows TSD to work early, thus allowing jemalloc and
libpthread to initialize together.

This incidentaly simplifies __pthread_enable/disable_asynccancel and
__pthread_self, now that ___pthread_self is always initialized.