ldbl-128ibm-compat: PLT redirects for using ldbl redirects internally

Tweak the PLT bypass magic when building glibc with long double
redirects.  This is made more difficult by the fact we only get
one chance to redirect functions.  This happens via the public
headers.

There are roughly three classes of redirect we need to attend to
today:

 1. Simple redirects, redirected via cdef macro overrides and
    and new libc_hidden_ldbl_proto macro.
 2. Internal usage of internal API, e.g __snprintf, which has
    no direct analogue.  This is bypassed directly on case-by-
    case basis.
 3. Double redirects, e.g sscanf and related.  These require
    a heavier handed approach of macro renaming to existing
    symbols.

Most simple redirects are handled via 1.  Ideally, the libc_*
macro would live in libc-symbols.h, but in practice the macros
needed for it to do anything useful live in cdefs.h, so they
are defined in the local override.

Notably, the internal name of the asprintf generated for ieee ldbl
redirects is renamed to work with internal prefixed usage.

This resolves the local plt usage introduced when building glibc
with ldbl == ieee128 on ppc64le.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>