Add support for custom loaders to the sanitizer symbolizer

Summary:
Adds a fallback mode to procmaps when the symbolizer
fails to locate a module for a given address by using
dl_iterate_phdr.

Reviewers: kubamracek, rnk, vitalybuka, eugenis

Reviewed By: eugenis

Subscribers: srhines, llvm-commits

Differential Revision: https://reviews.llvm.org/D37269

llvm-svn: 314431