powerpc64le: enforce non-specific long double in .gnu.attributes section

powerpc64le: enforce non-specific long double in .gnu.attributes section

We turn off this feature to avoid polluting our shared libary with
a specific value.  However, static libgcc is not under our control,
and has enabled this for ibm128 routines.  This pollutes the
resulting shared libraries with it.

Attach a post-linking hook to replace this section with one crafted
as hard-float + indeterminate ldbl.  This allows IEEE ldbl users to
avoid having to disable the gnu attributes feature which should
protect them from linking ibm ldbl libraries using the gnu attributes
feature.

Currently, this only replaces libc and libm which support both ldbl
formats and rely on application code to explicitly determine which
is to be used.

Strictly speaking, the section could be deleted with minimal lost value.
However correctly set attributes could prove useful for some future change,
and similarly missing attributes.

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