c++, v3: Emit fundamental tinfos for _Float16/decltype(0.0bf16) types on ia32 with -mno-sse2 [PR108883]

_Float16 and decltype(0.0bf16) types are on x86 supported only with
-msse2.  On x86_64 that is the default, but on ia32 it is not.
We should still emit fundamental type tinfo for those types in
libsupc++.a/libstdc++.*, regardless of whether libsupc++/libstdc++
is compiled with -msse2 or not, as user programs can be compiled
with different ISA flags from libsupc++/libstdc++ and if they
are compiled with -msse2 and use std::float16_t or std::bfloat16_t
and need RTTI for it, it should work out of the box.  Furthermore,
libstdc++ ABI on ia32 shouldn't depend on whether the library
is compiled with -mno-sse or -msse2.

Unfortunately, just hacking up libsupc++ Makefile/configure so that
a single source is compiled with -msse2 isn't appropriate, because
that TU emits also code and the code should be able to run on CPUs
which libstdc++ supports.  We could add [[gnu::attribute ("no-sse2")]]
there perhaps conditionally, but it all gets quite ugly.

The following patch instead adds a target hook which allows the backend
to temporarily tweak registered types such that emit_support_tinfos
emits whatever is needed.

Additionally, it makes emit_support_tinfos_1 call emit_tinfo_decl
immediately, so that temporarily created dummy types for emit_support_tinfo
purposes only can be nullified again afterwards.  And removes the
previous fallback_* types used for dfloat*_type_node tinfos even when
decimal types aren't supported.

2023-03-03  Jakub Jelinek  <jakub@redhat.com>

	PR target/108883
gcc/
	* target.h (emit_support_tinfos_callback): New typedef.
	* targhooks.h (default_emit_support_tinfos): Declare.
	* targhooks.cc (default_emit_support_tinfos): New function.
	* target.def (emit_support_tinfos): New target hook.
	* doc/tm.texi.in (emit_support_tinfos): Document it.
	* doc/tm.texi: Regenerated.
	* config/i386/i386.cc (ix86_emit_support_tinfos): New function.
	(TARGET_EMIT_SUPPORT_TINFOS): Redefine.
gcc/cp/
	* cp-tree.h (enum cp_tree_index): Remove CPTI_FALLBACK_DFLOAT*_TYPE
	enumerators.
	(fallback_dfloat32_type, fallback_dfloat64_type,
	fallback_dfloat128_type): Remove.
	* rtti.cc (emit_support_tinfo_1): If not emitted already, call
	emit_tinfo_decl and remove from unemitted_tinfo_decls right away.
	(emit_support_tinfos): Move &dfloat*_type_node from fundamentals array
	into new fundamentals_with_fallback array.  Call emit_support_tinfo_1
	on elements of that array too, with the difference that if
	the type is NULL, use a fallback REAL_TYPE for it temporarily.
	Drop the !targetm.decimal_float_supported_p () handling.  Call
	targetm.emit_support_tinfos at the end.
	* mangle.cc (write_builtin_type): Remove references to
	fallback_dfloat*_type.  Handle bfloat16_type_node mangling.

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