fix build regression on platforms with no native 64-bit CAS

Unsafe.compareAndSwapLong was moved from classpath-openjdk.cpp to
builtin.cpp, but the fieldForOffset helper function was not, which
only caused problems when I tried to build for ARM.  This commit moves
said helper function, along with Unsafe.getVolatileLong, which also
uses it.
fix iOS 7 build using latest Xcode

It looks like the iOS 7 SDK doesn't have GCC anymore, so we need to
use clang instead.  Also, thread_act.h and thread_status.h have moved,
so I updated arm.h accordingly.  That might break the build for older
SDKs, but I don't have one available at the moment.  If it does break,
I'll fix it.