Prepare redirections for IEEE long double on powerpc64le

Prepare redirections for IEEE long double on powerpc64le

All functions that have a format string, which can consume a long double
argument, must have one version for each long double format supported on
a platform.  On powerpc64le, these functions currently have two versions
(i.e.: long double with the same format as double, and long double with
IBM Extended Precision format).  Support for a third long double format
option (i.e. long double with IEEE long double format) is being prepared
and all the aforementioned functions now have a third version (not yet
exported on the master branch, but the code is in).

For these functions to get selected (during build time), references to
them in user programs (or dependent libraries) must get redirected to
the aforementioned new versions of the functions.  This patch installs
the header magic required to perform such redirections.

Notice, however, that since the redirections only happen when
__LONG_DOUBLE_USES_FLOAT128 is set to 1, and no platform (including
powerpc64le) currently does it, no redirections actually happen.
Redirections and the exporting of the new functions will happen at the
same time (when powerpc64le adds ldbl-128ibm-compat to their Implies.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>
Reviewed-by: Paul E. Murphy <murphyp@linux.vnet.ibm.com>