[scudo] Eliminate the runtime dependency on libc++abi

[phosek]

Summary

Making the CPU features variable non-static avoids the need for locking to ensure that the initialization is thread-safe which in turns eliminates the runtime dependency on libc++abi library (for cxa_guard_acquire and cxa_guard_release) which makes it possible to link scudo against pure C programs.

[EricWF]

Herald added me as a reviewer because libc++ was in the title, but this looks OK to me.

If you want to continue stashing the results of getCPUFeatures() without requiring runtime guards I imagine something like this would work (At least in optimized builds):

bool testCPUFeatures(CPUFeature Feat) {
  static CPUIDRegs FeaturesRegs = {}; // shouldn't generate a guard.
  static bool RegsInit = false; // also shouldn't generate a guard.
  if (!RegsInit) {
    FeatureRegs = getCPUFeatures();; // Not thread safe but initialization doesn't depend on libc++abi.
  }
  // ...
}

[cryptoad]

Since testCPUFeature is only used once, I'm be OK with the initial way.
Initially it was used multiple times hence it being stored statically, but it's no longer the case.
@phosek you probably want to check D28574 as well.

compiler-rt/trunk/lib/scudo/scudo_utils.cpp

 }
 
 #ifndef bit_SSE4_2
 #define bit_SSE4_2 bit_SSE42  // clang and gcc have different defines.
 #endif
 
 bool testCPUFeature(CPUFeature Feature)
 {
-  static CPUIDRegs FeaturesRegs = getCPUFeatures();
+  CPUIDRegs FeaturesRegs = getCPUFeatures();
 
   switch (Feature) {
     case CRC32CPUFeature:  // CRC32 is provided by SSE 4.2.
       return !!(FeaturesRegs.Ecx & bit_SSE4_2);
     default:
       break;
   }
   return false;