libstdc++: Add _Float128 to_chars/from_chars support for x86, ia64 and ppc64le with...

libstdc++: Add _Float128 to_chars/from_chars support for x86, ia64 and ppc64le with glibc

The following patch adds std::{to,from}_chars support for std::float128_t
on glibc 2.26+ for {i?86,x86_64,ia64,powerpc64le}-linux.
When long double is already IEEE quad, previous changes already handle
it by using long double overloads in _Float128 overloads.
The powerpc64le case (with explicit or implicit -mabi=ibmlongdouble)
is handled by using the __float128/__ieee128 entrypoints which are
already in the library and used for -mabi=ieeelongdouble.
For i?86, x86_64 and ia64 this patch adds new library entrypoints,
mostly by enabling the code that was already there for powerpc64le-linux.
Those use __float128 or __ieee128, the patch uses _Float128 for the
exported overloads and internally as template parameter.  While
powerpc64le-linux uses __sprintfieee128 and __strtoieee128,
for _Float128 the patch uses the glibc 2.26 strfromf128 and strtof128
APIs.  So that one can build gcc against older glibc and then compile
user programs on newer glibc, the patch uses weak references unless
gcc is compiled against glibc 2.26+.  strfromf128 unfortunately can't
handle %.0Lf and %.*Le, %.*Lf, %.*Lg format strings sprintf/__sprintfieee128
use, we need to remove the L from those and replace * with actually
directly printing the precision into the format string (i.e. it can
handle %.0f and %.27f (floating point type is implied from the function
name)).
Unlike the std::{,b}float16_t support, this one actually exports APIs
with std::float128_t aka _Float128 in the mangled name, because no
standard format is superset of it.  On the other side, e.g. on i?86/x86_64
it doesn't have restrictions like for _Float16/__bf16 which ISAs need
to be enabled in order to use it.

The denorm_min case in the testcase is temporarily commented out because
of the ERANGE subnormal issue Patrick posted patch for.

2022-11-07  Jakub Jelinek  <jakub@redhat.com>

	* include/std/charconv (from_chars, to_chars): Add _Float128
	overfloads if _GLIBCXX_HAVE_FLOAT128_MATH is defined.
	* config/abi/pre/gnu.ver (GLIBCXX_3.4.31): Export
	_ZSt8to_charsPcS_DF128_, _ZSt8to_charsPcS_DF128_St12chars_format,
	_ZSt8to_charsPcS_DF128_St12chars_formati and
	_ZSt10from_charsPKcS0_RDF128_St12chars_format.
	* src/c++17/floating_from_chars.cc (USE_STRTOF128_FOR_FROM_CHARS):
	Define if needed.
	(__strtof128): Declare.
	(from_chars_impl): Handle _Float128.
	(from_chars): New _Float128 overload if USE_STRTOF128_FOR_FROM_CHARS
	is define.
	* src/c++17/floating_to_chars.cc (__strfromf128): Declare.
	(FLOAT128_TO_CHARS): Define even when _Float128 is supported and
	wider than long double.
	(F128_type): Use _Float128 for that case.
	(floating_type_traits): Specialize for F128_type rather than
	__float128.
	(sprintf_ld): Add length argument.  Handle _Float128.
	(__floating_to_chars_shortest, __floating_to_chars_precision):
	Pass length to sprintf_ld.
	(to_chars): Add _Float128 overloads for the F128_type being
	_Float128 cases.
	* testsuite/20_util/to_chars/float128_c++23.cc: New test.