[stm32l1] Add semihosting example

This is not specifically an L1 example, but it lives beside the L1 "usart"
example to show how easy it is to add semihosting support to existing code.

Tested with the by now relatively old g-a-e 2012q4 release and
OpenOcd 0.8.0-dev-00011-g70a2ffa (2013-05-14-19:41)