RFC: powerpc64le: Enable support for IEEE long double

RFC: powerpc64le: Enable support for IEEE long double

Not for glibc-2.31 - Do not commit!

No changes since v3.

Changes since v2:

  - Added definition of LDBL_IBM128_COMPAT_VERSION and
    LDBL_IBM128_VERSION (moved from a previous commit).

Changes since v1:

  - Use __LONG_DOUBLE_USES_FLOAT128 directly.

-- 8< --
On platforms where long double may have two different formats, i.e.: the
same format as double (64-bits) or something else (128-bits), building
with -mlong-double-128 is the default and function calls in the user
program match the name of the function in Glibc.  When building with
-mlong-double-64, Glibc installed headers redirect such calls to the
appropriate function.

This patch adds similar redirections to be used by user code builds in
IEEE long double mode (-mabi=ieeelongdouble).  It also skips some uses
of libc_hidden_proto in internal headers, because they also produce
redirections, causing a redirection conflict.

PS: Missing NEWS entry.