riscv-trap-handling: Add functionality, prevent stack corruption

Trap handling code was bugged in that it loaded in the wrong stack
pointer, overwriting the space the processor uses to talk to its host
for doing device requests. Fix this issue, as well as add support for
handling misaligned loads the same way we handle misaligned stores.

Change-Id: I68ba3a114b7167b3212bb0bed181a7595f0b97d8
Signed-off-by: Thaminda Edirisooriya <thaminda@google.com>
Reviewed-on: http://review.coreboot.org/11620
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/arch/riscv/include/arch/exception.h

@@ -55,7 +55,7 @@ static inline void exception_init(void)
 
 void trap_handler(trapframe* tf);
 void handle_supervisor_call(trapframe* tf);
-//void handleMisalignedLoad(trapframe *tf);
+void handleMisalignedLoad(trapframe *tf);
 void handle_misaligned_store(trapframe *tf);
 
 #endif