[XRay][compiler-rt] Add another work-around to XRay FDR tests when TSC emulation is needed

[dyung]

Summary

This patch implements the work-around suggested by Dean in PR32620 comment 1 to fix the problem in that bug. The only difference is that the change was required in two places, not just one.

With this patch, the test "XRay-x86_64-linux :: TestCases/Linux/fdr-mode.cc" should now pass on a x86_64 linux machine that does not have support for rdtscp.

[dberris]

LGTM

@kpw -- just as note, we may need to introduce some logic in the initialisation routine for FDR mode to do the feature probing once.

@pelikan -- this use-case in FDR logging may introduce a wrinkle in the refactoring we talked about for cleaning up the emulation/raw tsc support.

lib/xray/xray_fdr_logging.cc

if (Fd == -1) {
     __sanitizer::atomic_store(&LogFlushStatus, Result,
                               __sanitizer::memory_order_release);
     return Result;
   }
 
   XRayFileHeader Header;
   Header.Version = 1;
   Header.Type = FileTypes::FDR_LOG;
-  Header.CycleFrequency = getTSCFrequency();
+  Header.CycleFrequency = probeRequiredCPUFeatures()
+                          ? getTSCFrequency() : __xray::NanosecondsPerSecond;
   // FIXME: Actually check whether we have 'constant_tsc' and 'nonstop_tsc'
   // before setting the values in the header.
   Header.ConstantTSC = 1;
   Header.NonstopTSC = 1;
   Header.FdrData = FdrAdditionalHeaderData{LocalBQ->ConfiguredBufferSize()};
   retryingWriteAll(Fd, reinterpret_cast<char *>(&Header),
                    reinterpret_cast<char *>(&Header) + sizeof(Header));
 

lib/xray/xray_fdr_logging_impl.h

if (EC != BufferQueue::ErrorCode::Ok) {
                                          __sanitizer::memory_order_acquire);
       if (LS != XRayLogInitStatus::XRAY_LOG_FINALIZING &&
           LS != XRayLogInitStatus::XRAY_LOG_FINALIZED)
         Report("Failed to acquire a buffer; error=%s\n",
                BufferQueue::getErrorString(EC));
       return;
     }
 
-    uint64_t CycleFrequency = getTSCFrequency();
+    uint64_t CycleFrequency = probeRequiredCPUFeatures()
+                              ? getTSCFrequency()
+                              : __xray::NanosecondsPerSecond;
     NumberOfTicksThreshold = CycleFrequency *
                              flags()->xray_fdr_log_func_duration_threshold_us /
                              1000000;
     setupNewBuffer(Buffer, wall_clock_reader);
   }
 
   if (CurrentCPU == std::numeric_limits<uint16_t>::max()) {
     // This means this is the first CPU this thread has ever run on. We set the