On FreeBSD, skip the first entry in the dl_iterate_phdr list.

Summary:
Similar to NetBSD, in FreeBSD, the first returned entry when callbacks
are done via dl_iterate_phdr will return the main program.  Ignore that
entry when checking that the dynamic ASan lib is loaded first.

Reviewers: eugenis, krytarowski, emaste, joerg

Reviewed By: eugenis, krytarowski

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D39253

llvm-svn: 316487