or1k fix bug: Initialize thread context with higher stack address.

Since or1k architecture stack grows down, threads should be initialized
with high stack addresses instead of lower ones. This is done in
_CPU_Context_Initialize function.
or1ksim: Console: implement uart_read_polled

Implement uart_read_polled which is needed for any application that
reads input from users. It works fine with pppd, capture, and all
termios tests.