linker: Refactoring step 3 of many

Extract linker executable specific code to linker_main.cpp;
this part of code does not have a place in libdl.a/so.

Bug: http://b/14998480
Bug: http://b/30706810
Test: mm && run bionic-unit-tests
Change-Id: I90f7475e93a919b0f9525da22928089ad35b8f6c
diff --git a/linker/Android.bp b/linker/Android.bp
index 39f1da9..4d770ac 100644
--- a/linker/Android.bp
+++ b/linker/Android.bp
@@ -22,6 +22,7 @@
         "linker_gdb_support.cpp",
         "linker_globals.cpp",
         "linker_libc_support.c",
+        "linker_main.cpp",
         "linker_namespaces.cpp",
         "linker_logger.cpp",
         "linker_mapped_file_fragment.cpp",