android: enable double-word CAS on x64
The android target assumes that for the x86_64 target, the CPU supports SSE4.2
and popcnt. This implies that the CPU is Nehalem or newer. This should be
sufficiently new to provide the double word compare and exchange instruction.
This allows us to directly lower __sync_val_compare_and_swap_16 to a cmpxchg16b.
It appears that the libatomic in android's NDK does not provide the
implementation for lowering calls to the library function.