www/qt5-webkit: fix build on powerpc

/wrkdirs/usr/ports/www/qt5-webkit/work/qtwebkit-5.212.0-alpha4/Source/WTF/wtf/Atomics.cpp:63:9: error: definition of builtin function 'sync_add_and_fetch_8'
int64_t sync_add_and_fetch_8(int64_t volatile* addend, int64_t value)

^

/wrkdirs/usr/ports/www/qt5-webkit/work/qtwebkit-5.212.0-alpha4/Source/WTF/wtf/Atomics.cpp:68:9: error: definition of builtin function 'sync_sub_and_fetch_8'
int64_t sync_sub_and_fetch_8(int64_t volatile* addend, int64_t value)