Replace CountOneBits and __builtin_popcount with POPCOUNT.

Clean up utils.h, make some functions constexpr.

Change-Id: I2399100280cbce81c3c4f5765f0680c1ddcb5883
diff --git a/runtime/arch/mips/context_mips.cc b/runtime/arch/mips/context_mips.cc
index b957708..0950e71 100644
--- a/runtime/arch/mips/context_mips.cc
+++ b/runtime/arch/mips/context_mips.cc
@@ -43,8 +43,8 @@
   mirror::ArtMethod* method = fr.GetMethod();
   uint32_t core_spills = method->GetCoreSpillMask();
   uint32_t fp_core_spills = method->GetFpSpillMask();
-  size_t spill_count = __builtin_popcount(core_spills);
-  size_t fp_spill_count = __builtin_popcount(fp_core_spills);
+  size_t spill_count = POPCOUNT(core_spills);
+  size_t fp_spill_count = POPCOUNT(fp_core_spills);
   size_t frame_size = method->GetFrameSizeInBytes();
   if (spill_count > 0) {
     // Lowest number spill is farthest away, walk registers and fill into context.