powerpc64le: Enable support for IEEE long double

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.

Likewise, the internals of glibc are now built against IEEE long double.
However, the only (minimally) notable usage of long double is difftime.

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