_REG_TPIDR
u->uc_mcontext.__gregs[_REG_TPIDR] = (__greg_t)(uintptr_t)private;
CTASSERT(offsetof(struct reg, r_tpidr) == _REG_TPIDR * sizeof(__greg_t));
mcp->__gregs[_REG_TPIDR] = (uintptr_t)l->l_private;
l->l_private = (void *)mcp->__gregs[_REG_TPIDR];