[tsan] Only intercept pthread_mutex_clocklock on Linux (#76220)

`tsan_interceptors_posix.cpp` doesn't compile on FreeBSD 14.0/amd64:
```
In file included from /vol/llvm/src/llvm-project/local-freebsd/compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp:25:
/vol/llvm/src/llvm-project/local-freebsd/compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp: In function ‘void __tsan::InitializeInterceptors()’:
/vol/llvm/src/llvm-project/local-freebsd/compiler-rt/lib/tsan/rtl/../../interception/interception.h:243:25: error: ‘real_pthread_mutex_clocklock’ is not a member of ‘__interception’; did you mean ‘real_pthread_mutex_unlock’?
```
Fixed by wrapping the `TSAN_INTERCEPT` invocation with `SANITIZER_LINUX`
as is already done for the interceptor definition.

Tested on `amd64-pc-freebsd14.0`.

Author: rorth

compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp

@@ -2918,7 +2918,9 @@ void InitializeInterceptors() {
   TSAN_INTERCEPT(pthread_mutex_trylock);
   TSAN_INTERCEPT(pthread_mutex_timedlock);
   TSAN_INTERCEPT(pthread_mutex_unlock);
+#if SANITIZER_LINUX
   TSAN_INTERCEPT(pthread_mutex_clocklock);
+#endif
 #if SANITIZER_GLIBC
 #  if !__GLIBC_PREREQ(2, 34)
   TSAN_INTERCEPT(__pthread_mutex_lock);