On FreeBSD use AT_EXECPATH from ELF auxiliary vectors for getExecutablePath

/proc/curproc/file and the KERN_PROC_PATHNAME sysctl may not return the
desired path if there are multiple hardlinks to the file, or if the path has
expired from the namecache.

Reviewed By:	theraven

Differential Revision:	https://reviews.llvm.org/D70198