Replace CountOneBits and __builtin_popcount with POPCOUNT.

Clean up utils.h, make some functions constexpr.

Change-Id: I2399100280cbce81c3c4f5765f0680c1ddcb5883
diff --git a/runtime/vmap_table.h b/runtime/vmap_table.h
index 2fbaebe..9821753 100644
--- a/runtime/vmap_table.h
+++ b/runtime/vmap_table.h
@@ -99,7 +99,7 @@
       }
       matches++;
     }
-    CHECK_LT(vmap_offset - matches, static_cast<uint32_t>(__builtin_popcount(spill_mask)));
+    CHECK_LT(vmap_offset - matches, static_cast<uint32_t>(POPCOUNT(spill_mask)));
     uint32_t spill_shifts = 0;
     while (matches != (vmap_offset + 1)) {
       DCHECK_NE(spill_mask, 0u);