[libc] Add fetch_or to the atomic interface

Summary:
This patch adds `fetch_or`. It is mainly useful for querying and set a
bit's status without using the more expensive `exchange` functions.

libc/src/__support/CPP/atomic.h

@@ -86,6 +86,10 @@ template <typename T> struct Atomic {
     return __atomic_fetch_add(&val, increment, int(mem_ord));
   }
 
+  T fetch_or(T mask, MemoryOrder mem_ord = MemoryOrder::SEQ_CST) {
+    return __atomic_fetch_or(&val, mask, int(mem_ord));
+  }
+
   T fetch_sub(T decrement, MemoryOrder mem_ord = MemoryOrder::SEQ_CST) {
     return __atomic_fetch_sub(&val, decrement, int(mem_ord));
   }