[bazel] Add preproccessed files as a type of side-product

Signed-off-by: Miguel Young de la Sota <mcyoung@google.com>
diff --git a/rules/opentitan.bzl b/rules/opentitan.bzl
index 4eb18c3..353a648 100644
--- a/rules/opentitan.bzl
+++ b/rules/opentitan.bzl
@@ -5,7 +5,13 @@
 # TODO(drewmacrae) this should be in rules_cc
 # pending resolution of https://github.com/bazelbuild/rules_cc/issues/75
 load("//rules:bugfix.bzl", "find_cc_toolchain")
-load("//rules:cc_side_outputs.bzl", "rv_asm", "rv_llvm_ir", "rv_relink_with_linkmap")
+load(
+    "//rules:cc_side_outputs.bzl",
+    "rv_asm",
+    "rv_llvm_ir",
+    "rv_preprocess",
+    "rv_relink_with_linkmap",
+)
 load(
     "//rules:rv.bzl",
     "rv_rule",
@@ -442,6 +448,13 @@
         **kwargs
     )
 
+    preproc_name = "{}_{}".format(name, "preproc")
+    targets.append(preproc_name)
+    rv_preprocess(
+        name = preproc_name,
+        target = name,
+    )
+
     asm_name = "{}_{}".format(name, "asm")
     targets.append(asm_name)
     rv_asm(