tsan: switch libignore from /proc/self/maps to dl_iterate_phdr

/proc/self/maps can't be read atomically, this leads to episodic
crashes in libignore as it thinks that a module is loaded twice.
See the new test for an example.
dl_iterate_phdr does not have this problem.
Switch libignore to dl_iterate_phdr.

llvm-svn: 287632