Prevent user-defined basename_r from breaking basename(3).

LP64 is immune because basename_r is hidden there, but on LP32 a basename_r
defined in the executable breaks basename because its call to basename_r
will resolve to that one rather than the one in libc.

Bug: http://b/22415484
Change-Id: Ied3ca7ad3fb0e744eb705fc924743f893b4ad490