Provide 8-byte Atomic::add fallback for 32-bit platforms

Currently, trying to use Atomic::add over 64-bit value on 32-bit platform results in:

/home/shade/jdk-jdk/build/linux-x86-normal-server-fastdebug/hotspot/variant-server/libjvm/objs/jfrStringPoolBuffer.o: In function `unsigned long long Atomic::FetchAndAdd<Atomic::PlatformAdd<8u> >::operator()<unsigned long long, unsigned long long>(unsigned long long, unsigned long long volatile*, atomic_memory_order) const':
/home/shade/jdk-jdk/src/hotspot/share/runtime/atomic.hpp:670: undefined reference to `unsigned long long Atomic::PlatformAdd<8u>::fetch_and_add<unsigned long long, unsigned long long>(unsigned long long, unsigned long long volatile*, atomic_memory_order) const'

This can be emulated with CAS-ed loop on most platforms.