ldbl-128ibm-compat: Add printf_size

ldbl-128ibm-compat: Add printf_size

Since the addition of the _Float128 API, strfromf128 and printf_size use
__printf_fp to print _Float128 values.  This is achieved by setting the
'is_binary128' member of the 'printf_info' structure to one.  Now that
the format of long double on powerpc64le is getting a third option, this
mechanism is reused for long double values that have binary128 format
(i.e.: when -mabi=ieeelongdouble).

This patch adds __printf_sizeieee128 as an exported symbol, but doesn't
provide redirections from printf_size, yet.  All redirections will be
installed in a future commit, once all other functions that print or
read long double values with binary128 format are ready.  In
__printf_fp, when 'is_binary128' is one, the floating-point argument is
treated as if it was of _Float128 type, regardless of the value of
'is_long_double', thus __printf_sizeieee128 sets 'is_binary128' to the
same value of 'is_long_double'.  Otherwise, double values would not be
printed correctly.

Tested for powerpc64le.