Make the linker relocatable.

Previously, the linker always loaded itself into the same
location in memory, which inhibited the effectiveness of Android's
ASLR implementation. Modify the linker code so it can be relocatable
and link itself at runtime.

Change-Id: I90d064743abdd29450ac0482ed28752b2196286c

linker/linker.h

@@ -83,6 +83,7 @@ typedef struct soinfo soinfo;
 #define FLAG_LINKED     0x00000001
 #define FLAG_ERROR      0x00000002
 #define FLAG_EXE        0x00000004 // The main executable
+#define FLAG_LINKER     0x00000010 // The linker itself
 
 #define SOINFO_NAME_LEN 128