Add exception vectors & low-level handlers

At the moment these are almost bit-for-bit copies of the vector
behaviour in the RTOS SDK, but hopefully that will start to change.

Progress towards #2

lib/libmain.remove

@@ -1,3 +1,4 @@
 # Object files to be removed from libmain
 printf-stdarg.o
-libc.o
+libc.o
+xtensa_vectors.o