[libc++abi] Provide __cxa_thread_atexit on Fuchsia

[phosek]

Summary

Fuchsia already supports this interface.

[mcgrathr]

I don't know what's affected by the presence of that declaration. Certainly it should be defined.
AFAICT this has nothing to do with solving the problem of the symbol not existing in the library.
That needs CMakeLists.txt:34 changed so it compiles cxa_thread_atexit.cpp for Fuchsia.
We also need to make sure that the cmake check for presence of __cxa_thread_atexit_impl in libc is passing correctly.

[phosek]

In D53801#1278430, @mcgrathr wrote:

I don't know what's affected by the presence of that declaration. Certainly it should be defined.
AFAICT this has nothing to do with solving the problem of the symbol not existing in the library.
That needs CMakeLists.txt:34 changed so it compiles cxa_thread_atexit.cpp for Fuchsia.

I missed that one.

We also need to make sure that the cmake check for presence of __cxa_thread_atexit_impl in libc is passing correctly.

I already checked that one and it's being detected properly.

[mcgrathr]

lgtm

libcxxabi/trunk/include/cxxabi.h

 __cxa_increment_exception_refcount(void *primary_exception) throw();
 extern _LIBCXXABI_FUNC_VIS void
 __cxa_decrement_exception_refcount(void *primary_exception) throw();
 
 // Apple extension to support std::uncaught_exception()
 extern _LIBCXXABI_FUNC_VIS bool __cxa_uncaught_exception() throw();
 extern _LIBCXXABI_FUNC_VIS unsigned int __cxa_uncaught_exceptions() throw();
 
-#ifdef __linux__
-// Linux TLS support. Not yet an official part of the Itanium ABI.
+#if defined(__linux__) || defined(__Fuchsia__)
+// Linux and Fuchsia TLS support. Not yet an official part of the Itanium ABI.
 // https://sourceware.org/glibc/wiki/Destructor%20support%20for%20thread_local%20variables
 extern _LIBCXXABI_FUNC_VIS int __cxa_thread_atexit(void (*)(void *), void *,
                                                    void *) throw();
 #endif
 
 } // extern "C"
 } // namespace __cxxabiv1
 
 namespace abi = __cxxabiv1;
 
 #endif // __cplusplus
 
 #endif // __CXXABI_H

libcxxabi/trunk/src/CMakeLists.txt

 
 if (LIBCXXABI_ENABLE_EXCEPTIONS)
   list(APPEND LIBCXXABI_SOURCES cxa_exception.cpp)
   list(APPEND LIBCXXABI_SOURCES cxa_personality.cpp)
 else()
   list(APPEND LIBCXXABI_SOURCES cxa_noexception.cpp)
 endif()
 
-if (LIBCXXABI_ENABLE_THREADS AND UNIX AND NOT (APPLE OR CYGWIN))
+if (LIBCXXABI_ENABLE_THREADS AND (UNIX OR FUCHSIA) AND NOT (APPLE OR CYGWIN))
   list(APPEND LIBCXXABI_SOURCES cxa_thread_atexit.cpp)
 endif()
 
 set(LIBCXXABI_HEADERS ../include/cxxabi.h)
 
 # Add all the headers to the project for IDEs.
 if (MSVC_IDE OR XCODE)
   # Force them all into the headers dir on MSVC, otherwise they end up at