[x265] [RESEND 2 2/2] source/test/testharness.h: don't redefine `__rdtsc()` builtin

[Sergei Trofimovich]

On darwin clang-16 provides `__rdtsc()` builtin. As a result the build
fails in `nixpkgs` as:

    source/test/testharness.h:78:24: error: static declaration of '__rdtsc' follows non-static declaration
    static inline uint32_t __rdtsc(void)
                           ^
    x265_3.6/source/test/testharness.h:78:24: note: '__rdtsc' is a builtin with type 2

The change avoid redefinition on targets that define `__rdtsc()` builtin.
---
 source/test/testharness.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/source/test/testharness.h b/source/test/testharness.h
index 5b8e274b1..357f4154a 100644
--- a/source/test/testharness.h
+++ b/source/test/testharness.h
@@ -69,6 +69,8 @@ protected:
 #include <intrin.h>
 #elif HAVE_RDTSC
 #include <intrin.h>
+#elif defined(__has_builtin) && __has_builtin(__rdtsc)
+/* compiler-provided builtin */
 #elif (!defined(__APPLE__) && (defined (__GNUC__) && (defined(__x86_64__) || defined(__i386__))))
 #include <x86intrin.h>
 #elif ( !defined(__APPLE__) && defined (__GNUC__) && defined(__ARM_NEON__))
-- 
2.46.0