Move some utility functions to linker_utils

Also adds unit-tests for page_start, page_offset, and safe_add

Change-Id: Ia1325b4682d367328a01599a19848e4ffcd2c0ea
diff --git a/linker/linker_utils.h b/linker/linker_utils.h
index b998fb5..65ffbdc5 100644
--- a/linker/linker_utils.h
+++ b/linker/linker_utils.h
@@ -24,4 +24,8 @@
 bool file_is_in_dir(const std::string& file, const std::string& dir);
 bool parse_zip_path(const char* input_path, std::string* zip_path, std::string* entry_path);
 
+off64_t page_start(off64_t offset);
+size_t page_offset(off64_t offset);
+bool safe_add(off64_t* out, off64_t a, size_t b);
+
 #endif