QtWebKit 5.212.0-alpha2 fails to build on PowerPC 32-bit (Big Endian)

With Glibc 2.26, and GCC 7.2.0. Here below is the build log (after the original build failed, and built again on the same tree to shorten the build log).

```

[  1%] Built target bmalloc
[  2%] Built target WTF
[  3%] Built target stageSharedScripts
[  3%] Built target LLIntOffsetsExtractor
[ 21%] Built target JavaScriptCore
[ 21%] Built target testb3
[ 21%] Built target jsc
[ 21%] Built target testair
[ 21%] Built target brotli
[ 22%] Built target woff2
[ 24%] Built target ANGLESupport
[ 81%] Built target WebCore
[ 82%] Built target WebCoreTestSupport
[ 82%] Built target gtest
[ 82%] Built target WebKit2-forwarding-headers
[ 82%] Automatic MOC for target WebKit2
[ 82%] Built target WebKit2_autogen
[ 97%] Built target WebKit2
[ 97%] Automatic MOC for target WebKit
[ 97%] Built target WebKit_autogen
[ 97%] Linking CXX shared library ../../lib/libQt5WebKit.so
../../lib/../Source/WebKit2/CMakeFiles/WebKit2.dir/Platform/IPC/Connection.cpp.o: In function `IPC::Connection::createSyncMessageEncoder(IPC::StringReference, IPC::StringReference, unsigned long long, unsigned long long&)':
Connection.cpp.text.ZN3IPC10Connection24createSyncMessageEncoderENS_15StringReferenceES1_yRy+0xbc): undefined reference to `_atomic_fetch_add_8'
../../lib/../Source/WebKit2/CMakeFiles/WebKit2.dir/Platform/IPC/Connection.cpp.o: In function `IPC::Connection::sendMessage(std::unique_ptr<IPC::MessageEncoder, std::default_delete<IPC::MessageEncoder> >, unsigned int, bool)':
Connection.cpp.text.ZN3IPC10Connection11sendMessageESt10unique_ptrINS_14MessageEncoderESt14default_deleteIS2_EEjb+0x354): undefined reference to `_atomic_fetch_add_8'
../../lib/../Source/WebKit2/CMakeFiles/WebKit2.dir/Platform/IPC/Connection.cpp.o: In function `IPC::Connection::sendSyncReply(std::unique_ptr<IPC::MessageEncoder, std::default_delete<IPC::MessageEncoder> >)':
Connection.cpp.text.ZN3IPC10Connection13sendSyncReplyESt10unique_ptrINS_14MessageEncoderESt14default_deleteIS2_EE+0x310): undefined reference to `_atomic_fetch_add_8'
../../lib/../Source/WebKit2/CMakeFiles/WebKit2.dir/UIProcess/StatisticsRequest.cpp.o: In function `WebKit::StatisticsRequest::addOutstandingRequest()':
StatisticsRequest.cpp.text.ZN6WebKit17StatisticsRequest21addOutstandingRequestEv+0x68): undefined reference to `_atomic_fetch_add_8'
StatisticsRequest.cpp.text.ZN6WebKit17StatisticsRequest21addOutstandingRequestEv+0x2d0): undefined reference to `_atomic_fetch_add_8'
collect2: error: ld returned 1 exit status
make[2]: *** [Source/WebKit/CMakeFiles/WebKit.dir/build.make:2646: lib/libQt5WebKit.so.5.212.0] Error 1
make[1]: *** [CMakeFiles/Makefile2:1053: Source/WebKit/CMakeFiles/WebKit.dir/all] Error 2
make: *** [Makefile:163: all] Error 2

```