Linux: Include <dl-auxv.h> in dl-sysdep.c only for SHARED

Otherwise, <dl-auxv.h> on POWER ends up being included twice,
once in dl-sysdep.c, once in dl-support.c.  That leads to a linker
failure due to multiple definitions of _dl_cache_line_size.

Fixes commit d96d2995
("Revert "Linux: Consolidate auxiliary vector parsing").