ldbl-128ibm-compat: Add ISO C99 versions of scanf functions

In the format string for *scanf functions, the '%as', '%aS', and '%a[]'
modifiers behave differently depending on ISO C99 compatibility.  When
_GNU_SOURCE is defined and -std=c89 is passed to the compiler, these
functions behave like ascanf, and the modifiers allocate memory for the
output.  Otherwise, the ISO C99 compliant version of these functions is
used, and the modifiers consume a floating-point argument.  This patch
adds the IEEE binary128 variant of ISO C99 compliant functions for the
third long double format on powerpc64le.

Tested for powerpc64le.

Reviewed-by: Paul E. Murphy <murphyp@linux.ibm.com>