While these glibc-introduced functions are, IMO, ugly and not really a
worthwhile addition to a libc interface, they do get used. So, in the
interests of compatibility with existing code, here's an
implementation...they're standalone, and pretty trivial, so it seems
reasonable to include, despite it being unfortunate that they exist at
all.

The added test case here passes with both musl and glibc.

A question: is the weak_alias stuff required in this case? I'm not
100% clear as to when that's required, and when it's not, but I think
perhaps it's not actually needed here, because nothing else in the
libc refers to these nonstandard symbols, and the file defines no
standard symbols?