Do not redirect calls to __GI_* symbols, when redirecting to *ieee128

No changes since v3.

No changes since v2.

Changes since v1:

  - Reduced the number of changes to the minimum required to build *cvt
    files in -mabi=ieeelongdouble mode.
  - Added __LONG_DOUBLE_USES_FLOAT128 to all long-double.h files.

-- 8< --
On platforms where long double has IEEE binary128 format as a third
option (initially, only powerpc64le), many exported functions are
redirected to their __*ieee128 equivalents.  This redirection is
provided by installed headers such as stdio-ldbl.h, and is supposed to
work correctly with user code.

However, during the build of glibc, similar redirections are employed,
in internal headers, such as include/stdio.h, in order to avoid extra
PLT entries.  These redirections conflict with the redirections to
__*ieee128, and must be avoided during the build.  This patch protects
the second redirections with a test for __LONG_DOUBLE_USES_FLOAT128, a
new macro that is defined to 1 when functions that deal with long double
typed values reuses the _Float128 implementation (this is currently only
true for powerpc64le).

Tested for powerpc64le, x86_64, and with build-many-glibcs.py.

Co-authored-by: Gabriel F. T. Gomes <gabrielftg@linux.ibm.com>
Reviewed-by: Florian Weimer <fweimer@redhat.com>