Drop os::is_MP checks from Atomics

Our Atomics are used in high-performance code within the VM, for example, garbage collectors. The performance of tight loops with atomics depends on the code quality within the loop. In our implementation, we have a LOCK_IF_MP stub that emits additional checks before actually invoking the atomic: this includes polling _processor_count, polling AssumeMP, several branches, etc:

// Adding a lock prefix to an instruction on MP machine
#define LOCK_IF_MP(mp) "cmp $0, " #mp "; je 1f; lock; 1: "

inline jint Atomic::add (jint add_value, volatile jint* dest) {
  jint addend = add_value;
  int mp = os::is_MP();
  __asm__ volatile ( LOCK_IF_MP(%3) "xaddl %0,(%2)"
                    : "=r" (addend)
                    : "0" (addend), "r" (dest), "r" (mp)
                    : "cc", "memory");
  return addend + add_value;
}

Since in 2016 we are mostly running heavily-threaded hardware, it might make sense to drop the LOCK_IF_MP from Atomics. (Additionally, since some time, x86 assumes lock prefix for some insns, like xchg, maybe we can drop the prefix altogether?)