stdlib: Do not use GLIBC_PRIVATE ABI for errno in libc_nonshared.a

Using TLS directly introduces a GLIBC_PRIVATE ABI dependency
into libc_nonshared.a, and thus indirectly into applications.
Adding the !defined LIBC_NONSHARED condition deactivates direct
TLS access, and libc_nonshared.a code switches to using
__errno_location, like application code.

Currently, this has no effect because there is no code in
libc_nonshared.a that accesses errno.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>