arm: implement safe reads by intercepting the data abort handler.

Implement functions to read from a memory location and indicate
failure in case this is not possible. Since we do not have a MMU,
intercept the data abort handler and simply return when the abort
comes from the safe read routines.

Change-Id: I08f2e59898dcac893319a8150d4cf626f3adabbd
Reviewed-on: http://gerrit.rockbox.org/207
Reviewed-by: Marcin Bukat <marcin.bukat@gmail.com>

lib/unwarminder/SOURCES

@@ -5,3 +5,4 @@ unwarm.c
 unwarminder.c
 unwarmmem.c
 unwarm_thumb.c
+safe_read.S