Move some utility functions to linker_utils

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

Change-Id: Ia1325b4682d367328a01599a19848e4ffcd2c0ea

linker/linker_utils.cpp

@@ -105,3 +105,23 @@ bool parse_zip_path(const char* input_path, std::string* zip_path, std::string*
   return true;
 }
 
+constexpr off64_t kPageMask = ~static_cast<off64_t>(PAGE_SIZE-1);
+
+off64_t page_start(off64_t offset) {
+  return offset & kPageMask;
+}
+
+bool safe_add(off64_t* out, off64_t a, size_t b) {
+  CHECK(a >= 0);
+  if (static_cast<uint64_t>(INT64_MAX - a) < b) {
+    return false;
+  }
+
+  *out = a + b;
+  return true;
+}
+
+size_t page_offset(off64_t offset) {
+  return static_cast<size_t>(offset & (PAGE_SIZE-1));
+}
+