FreeBSD's libstdc++ build (used on tier-2 architectures) uses GNU ld's 
-f <name> option, which sets the DT_AUXILIARY field to the specified name.
Multiple -f options may be specified and the DT_AUXILIARY entries 
will be added in the order in which they appear.

Patch implements that option.

Differential revision: https://reviews.llvm.org/D24139

llvm-svn: 280475