ART: Fix casts in atomic.h to please GCC

GCC 4.9 for ARM64 is not happy about reinterpret_cast-ing between
int64_t and uint64_t, which is according to spec. Change the
concrete casts to static_cast.

However, we also use this for pointers, and we cannot static_cast
those to int64_t. So add a reinterpret_cast to uintptr_t.

Change-Id: If6513fbcbb8ee8f610f172310af61cf2e9ab0c43