[asan] Always skip first object from dl_iterate_phdr

All platforms return the main executable as the first dl_phdr_info.
FreeBSD, NetBSD, Solaris, and Linux-musl place the executable name
in the dlpi_name field of this entry. It appears that only Linux-glibc
uses the empty string.

To make this work generically on all platforms, unconditionally
skip the first object (like is currently done for FreeBSD and NetBSD).
This fixes first DSO detection on Linux-musl. It also would likely
fix detection on Solaris/Illumos if it were to gain PIE support
(since dlpi_addr would not be NULL).

Additionally, only skip the Linux VDSO on linux.

Finally, use the empty string as the "seen first dl_phdr_info"
marker rather than (char *)-1. If there was no other object, we
would try to dereference it for a string comparison.

Reviewed By: MaskRay, vitalybuka

Differential Revision: https://reviews.llvm.org/D119515