i386, rs6000, ia64, s390: Fix C++ ICEs with _Float64x or _Float128 [PR107080]

i386, rs6000, ia64, s390: Fix C++ ICEs with _Float64x or _Float128 [PR107080]

The following testcase ICEs on x86 as well as ppc64le (the latter
with -mabi=ieeelongdouble), because _Float64x there isn't mangled as
DF64x but e or u9__ieee128 instead.
Those are the mangling that should be used for the non-standard
types with the same mode or for long double, but not for _Float64x.
All the 4 mangle_type targhook implementations start with
type = TYPE_MAIN_VARIANT (type);
so I think it is cleanest to handle it the same in all and return NULL
before the switches on mode or whatever other tests.
s390 doesn't actually have a bug, but while I was there, having
type = TYPE_MAIN_VARIANT (type);
if (TYPE_MAIN_VARIANT (type) == long_double_type_node)
looked useless to me.

Note, there is one further problem on aarch64/arm, types with HFmode
(_Float16 and __fp16) are there mangled as Dh (which is standard
Itanium mangling:
                 ::= Dh # IEEE 754r half-precision floating point (16 bits)
                 ::= DF <number> _ # ISO/IEC TS 18661 binary floating point type _FloatN (N bits)
so in theory is also ok, but DF16_ is more specific.  Should we just
change Dh to DF16_ in those backends, or should __fp16 there be distinct
type from _Float16 where __fp16 would mangle Dh and _Float16 DF16_ ?
And there is csky, which mangles __fp16 (but only if type's name is __fp16,
not _Float16) as __fp16, that looks clearly invalid to me as it isn't
valid in the mangling grammar.  So perhaps just nuke csky's mangle_type
and have it mangled as DF16_ by the generic code?

2022-09-30  Jakub Jelinek  <jakub@redhat.com>

	PR c++/107080
	* config/i386/i386.cc (ix86_mangle_type): Always return NULL
	for float128_type_node or float64x_type_node, don't check
	float128t_type_node later on.
	* config/ia64/ia64.cc (ia64_mangle_type): Always return NULL
	for float128_type_node or float64x_type_node.
	* config/rs6000/rs6000.cc (rs6000_mangle_type): Likewise.
	Don't check float128_type_node later on.
	* config/s390/s390.cc (s390_mangle_type): Don't use
	TYPE_MAIN_VARIANT on type which was set to TYPE_MAIN_VARIANT
	a few lines earlier.

	* g++.dg/cpp23/ext-floating11.C: New test.