samples: tracing: Add a sample for tracing_user

The user tracing backend allows one to implement weak symbols to perform
user specific tracing, e.g gpio toggling or spi transactions.  This adds
a qemu sample that just does a simple printk for each of the traced
functions, but could be changed to do whatever function the user
desires.

Signed-off-by: Bradley Bolen <bbolen@lexmark.com>