Add strfromf128 to stdlib when _Float128 support is enabled.
* stdio-common/printf-parsemb.c (__parse_one_specmb): Initialize
spec->info.is_binary128 to zero.
* stdio-common/printf.h (printf_info): Add new member is_binary128
to indicate that the number being converted to string is compatible
with the IEC 60559 binary128 format.
* stdio-common/printf_fp.c (__printf_fp_l): Add code to deal with
_Float128 numbers.
* stdio-common/printf_fphex.c: Include ieee754_float128.h and
ldbl-128/printf_fphex_macros.h
(__printf_fphex): Add code to deal with _Float128 numbers.
* stdio-common/printf_size.c (__printf_size): Likewise.
* stdio-common/vfprintf.c (process_arg): Initialize member
info.is_binary128 to zero.
* stdlib/fpioconst.h (FLT128_MAX_10_EXP_LOG): New macro.
* stdlib/stdlib.h: Include bits/floatn.h for _Float128 support.
(strfromf128): New declaration.
* stdlib/strfrom-skeleton.c (STRFROM): Set member info.is_binary128
to one.
* sysdeps/ieee754/float128/Makefile: Add strfromf128.
* sysdeps/ieee754/float128/Versions: Likewise.
* sysdeps/ieee754/float128/strfromf128.c: New file.
