Removed field `t_stacktop`still referenced in a few places

[ccollins476ad]

t_stacktop was recently removed from struct os_task. The removed field is still referenced in a few places, causing build errors for certain configurations.

./repos/apache-mynewt-core/kernel/os/src/os_stacktrace.c:87:        if (sp > (uintptr_t)t->t_stacktop && end > (uintptr_t)t->t_stacktop) {
./repos/apache-mynewt-core/kernel/os/src/os_stacktrace.c:88:            end = (uintptr_t)t->t_stacktop;
./repos/apache-mynewt-core/kernel/os/src/os_sched.c:83:    top = g_current_task->t_stacktop - g_current_task->t_stacksize;

[andrzej-kaczmarek]

I am not sure how I missed those when greping through source code... here's the fix: https://github.com/apache/mynewt-core/pull/2123