[XRay][SystemZ] Use stckf for non-clang compilers

Turns out there are users who use gcc to compile compiler-rt. Using the clang-specific builtin function `__builtin_readcyclecounter()` does not work in this case.
Solution is to use inline assembly using the stckf instruction in case the compiler is not clang.

Author: redstar

compiler-rt/lib/xray/xray_tsc.h

@@ -96,7 +96,13 @@ namespace __xray {
 inline bool probeRequiredCPUFeatures() XRAY_NEVER_INSTRUMENT { return true; }
 
 ALWAYS_INLINE uint64_t readTSC(uint8_t &CPU) XRAY_NEVER_INSTRUMENT {
+#if defined(__clang__)
   return __builtin_readcyclecounter();
+#else
+  uint64_t Cycles;
+  asm volatile("stckf %0" : /* No output. */ : "Q"(Cycles) : "cc");
+  return Cycles;
+#endif
 }
 
 inline uint64_t getTSCFrequency() XRAY_NEVER_INSTRUMENT {