linker: remove soinfo::entry

This field is used only once and only for main executable;
no need to carry it around in soinfo structure.

Change-Id: I7f80c64e0939e28966b6fed858891f9ce2a9e26b
diff --git a/linker/linker.h b/linker/linker.h
index 389c5b3..c669c46 100644
--- a/linker/linker.h
+++ b/linker/linker.h
@@ -173,7 +173,9 @@
  public:
   const ElfW(Phdr)* phdr;
   size_t phnum;
-  ElfW(Addr) entry;
+#if defined(__work_around_b_24465209__)
+  ElfW(Addr) unused0; // DO NOT USE, maintained for compatibility.
+#endif
   ElfW(Addr) base;
   size_t size;