Replace CountOneBits and __builtin_popcount with POPCOUNT.

Clean up utils.h, make some functions constexpr.

Change-Id: I2399100280cbce81c3c4f5765f0680c1ddcb5883
diff --git a/runtime/arch/x86/context_x86.cc b/runtime/arch/x86/context_x86.cc
index 5cf3001..c68d76a 100644
--- a/runtime/arch/x86/context_x86.cc
+++ b/runtime/arch/x86/context_x86.cc
@@ -38,7 +38,7 @@
 void X86Context::FillCalleeSaves(const StackVisitor& fr) {
   mirror::ArtMethod* method = fr.GetMethod();
   uint32_t core_spills = method->GetCoreSpillMask();
-  size_t spill_count = __builtin_popcount(core_spills);
+  size_t spill_count = POPCOUNT(core_spills);
   DCHECK_EQ(method->GetFpSpillMask(), 0u);
   size_t frame_size = method->GetFrameSizeInBytes();
   if (spill_count > 0) {