pw_cpu_exception: Add CPU exception module

This change introduces pw_cpu_exception, a facade that provides a
high-level interface for handling CPU exceptions.

Change-Id: I1f98ac82dd54040448b2e2ac32a392c49cb6dfba
diff --git a/pw_vars_default.gni b/pw_vars_default.gni
index c8cb803..baec2b4 100644
--- a/pw_vars_default.gni
+++ b/pw_vars_default.gni
@@ -69,3 +69,6 @@
 
 # Backend for the pw_dumb_io module.
 dir_pw_dumb_io_backend = ""
+
+# Backend for the pw_cpu_exception module.
+dir_pw_cpu_exception_backend = ""