linker: stat /proc/self/exe instead of executable_path

The absolute path to an executable may no longer be valid for
example when the file is unlinked immediately after exec.
Using /proc/self/exe instead of absolute path solves this problem.

Bug: http://b/31084669
Test: Run the app from http://b/31084669 make sure executable starts
Test: by checking ps and /proc/<pid>/exe
Change-Id: I5c819f39ef0fc4fc71b05de71e8af9ede611f04c
(cherry picked from commit 7da4bbbe)