ldbl-128ibm-compat: Add strfmon_l with IEEE long double format

Similarly to what has been done for printf-like functions, more
specifically to the internal implementation in __vfprintf_internal, this
patch extends __vstrfmon_l_internal to deal with long double values with
binary128 format (as a third format option and reusing the float128
implementation).

Tested for powerpc64le, powerpc64, x86_64, and with build-many-glibcs.

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