refactoring: introduce MappedFileFragment

Change-Id: I97b0c1131711ed0ac13ab33e3357e99e0b6b1a4e

linker/linker_debug.h

@@ -58,6 +58,13 @@
 
 __LIBC_HIDDEN__ extern int g_ld_debug_verbosity;
 
+#define CHECK(predicate) { \
+    if (!(predicate)) { \
+      __libc_fatal("%s:%d: %s CHECK '" #predicate "' failed", \
+          __FILE__, __LINE__, __FUNCTION__); \
+    } \
+  }
+
 #if LINKER_DEBUG_TO_LOG
 #define _PRINTVF(v, x...) \
     do { \