Replace CountOneBits and __builtin_popcount with POPCOUNT.

Clean up utils.h, make some functions constexpr.

Change-Id: I2399100280cbce81c3c4f5765f0680c1ddcb5883
diff --git a/runtime/arch/arm/fault_handler_arm.cc b/runtime/arch/arm/fault_handler_arm.cc
index 3bbec71..eddaa0b 100644
--- a/runtime/arch/arm/fault_handler_arm.cc
+++ b/runtime/arch/arm/fault_handler_arm.cc
@@ -233,9 +233,9 @@
 
   mirror::ArtMethod* method = reinterpret_cast<mirror::ArtMethod*>(sc->arm_r0);
   uint32_t spill_mask = method->GetCoreSpillMask();
-  uint32_t numcores = __builtin_popcount(spill_mask);
+  uint32_t numcores = POPCOUNT(spill_mask);
   uint32_t fp_spill_mask = method->GetFpSpillMask();
-  uint32_t numfps = __builtin_popcount(fp_spill_mask);
+  uint32_t numfps = POPCOUNT(fp_spill_mask);
   uint32_t spill_size = (numcores + numfps) * 4;
   LOG(DEBUG) << "spill size: " << spill_size;
   uint8_t* prevframe = prevsp + spill_size;