Add LLVM libcxxabi (#1063)

* third_party: Add libcxxabi

Added libcxxabi from LLVM 17.0.6
The library implements the Itanium C++ exception handling ABI.

* third_party/libcxxabi: Enable __cxa_thread_atexit

Enable `__cxa_thread_atexit` from libcxxabi.
`__cxa_thread_atexit_impl` is still implemented by the cosmo libc.
The original `__cxa_thread_atexit` has been removed.

* third_party/libcxx: Build with exceptions

Build libcxx with exceptions enabled.

- Removed `_LIBCPP_NO_EXCEPTIONS` from `__config`.
- Switched the exception implementation to `libcxxabi`. These two files
are taken from the same `libcxx` version as mentioned in `README.cosmo`.
- Removed `new_handler_fallback` in favor of `libcxxabi` implementation.
- Enable `-fexceptions` and `-frtti` for `libcxx`.
- Removed `THIRD_PARTY_LIBCXX` dependency from `libcxxabi` and
`libunwind`. These libraries do not use any runtime `libcxx` functions,
just headers.

* libc: Remove remaining redundant cxa functions

- `__cxa_pure_virtual` in `libcxxabi` is also a stub similar to the
existing one.
- `__cxa_guard_*` from `libcxxabi` is used instead of the ones from
Android.

Now there should be no more duplicate implementations.
`__cxa_thread_atexit_impl`, `__cxa_atexit`, and related supporting
functions, are still left to other libraries as in `libcxxabi`.

`libcxxabi` is also now added to `cosmopolitan.a` to make up for the
removed functions.

Affected in-tree libraries (`third_party/double-conversion`) have been
updated.

third_party/libcxxabi/demangle/ItaniumNodes.def

@@ -0,0 +1,95 @@
+//===------------------------- ItaniumNodes.def ----------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Define the demangler's node names
+
+#ifndef NODE
+#error Define NODE to handle nodes
+#endif
+
+NODE(NodeArrayNode)
+NODE(DotSuffix)
+NODE(VendorExtQualType)
+NODE(QualType)
+NODE(ConversionOperatorType)
+NODE(PostfixQualifiedType)
+NODE(ElaboratedTypeSpefType)
+NODE(NameType)
+NODE(AbiTagAttr)
+NODE(EnableIfAttr)
+NODE(ObjCProtoName)
+NODE(PointerType)
+NODE(ReferenceType)
+NODE(PointerToMemberType)
+NODE(ArrayType)
+NODE(FunctionType)
+NODE(NoexceptSpec)
+NODE(DynamicExceptionSpec)
+NODE(FunctionEncoding)
+NODE(LiteralOperator)
+NODE(SpecialName)
+NODE(CtorVtableSpecialName)
+NODE(QualifiedName)
+NODE(NestedName)
+NODE(LocalName)
+NODE(ModuleName)
+NODE(ModuleEntity)
+NODE(VectorType)
+NODE(PixelVectorType)
+NODE(BinaryFPType)
+NODE(BitIntType)
+NODE(SyntheticTemplateParamName)
+NODE(TypeTemplateParamDecl)
+NODE(NonTypeTemplateParamDecl)
+NODE(TemplateTemplateParamDecl)
+NODE(TemplateParamPackDecl)
+NODE(ParameterPack)
+NODE(TemplateArgumentPack)
+NODE(ParameterPackExpansion)
+NODE(TemplateArgs)
+NODE(ForwardTemplateReference)
+NODE(NameWithTemplateArgs)
+NODE(GlobalQualifiedName)
+NODE(ExpandedSpecialSubstitution)
+NODE(SpecialSubstitution)
+NODE(CtorDtorName)
+NODE(DtorName)
+NODE(UnnamedTypeName)
+NODE(ClosureTypeName)
+NODE(StructuredBindingName)
+NODE(BinaryExpr)
+NODE(ArraySubscriptExpr)
+NODE(PostfixExpr)
+NODE(ConditionalExpr)
+NODE(MemberExpr)
+NODE(SubobjectExpr)
+NODE(EnclosingExpr)
+NODE(CastExpr)
+NODE(SizeofParamPackExpr)
+NODE(CallExpr)
+NODE(NewExpr)
+NODE(DeleteExpr)
+NODE(PrefixExpr)
+NODE(FunctionParam)
+NODE(ConversionExpr)
+NODE(PointerToMemberConversionExpr)
+NODE(InitListExpr)
+NODE(FoldExpr)
+NODE(ThrowExpr)
+NODE(BoolExpr)
+NODE(StringLiteral)
+NODE(LambdaExpr)
+NODE(EnumLiteral)
+NODE(IntegerLiteral)
+NODE(FloatLiteral)
+NODE(DoubleLiteral)
+NODE(LongDoubleLiteral)
+NODE(BracedExpr)
+NODE(BracedRangeExpr)
+
+#undef NODE