https://bugs.gentoo.org/786549

gcc-11 defines __rdtsc as a macro:
    // include/ia32intrin.h:110
    #define __rdtsc() __builtin_ia32_rdtsc ()
and causes build failure:
    intrincs/rdtsc.c:15:30: error: macro "__rdtsc" passed 1 arguments, but takes just 0
       15 | unsigned __int64 __rdtsc(void)
          |                              ^

Let's avoid __rdtsc definition on systems with #define __rdtsc present.

There is still a chance that it might be a '#define __rdtsc __rdtsc'.
We'll revisit it then.
--- a/mingw-w64-crt/intrincs/rdtsc.c
+++ b/mingw-w64-crt/intrincs/rdtsc.c
@@ -11,7 +11,7 @@
   #define __has_builtin(x) 0
 #endif
 
-#if !__has_builtin(__rdtsc)
+#if !__has_builtin(__rdtsc) && !defined(__rdtsc)
 unsigned __int64 __rdtsc(void)
 {
 #ifdef _WIN64