Make pthread's __libcpp_get_tls declaration consistent

[pirama]

Summary

Add the _LIBCPP_TLS_DESTRUCTOR_CC attribute to the at_exit parameter
for the pthread definition of libcpp_get_tls. This makes it
consistent with the rest (the non-pthread declaration in
include/__threading_support, and calls in include/thread and
srcsrc/support/win32/thread_win32.cpp).

This has NFC because _LIBCPP_TLS_DESTRUCTOR_CC is empty when pthread is
available.

[pirama]

Ping...

include/__threading_support

else
      __ts.tv_sec = __ts_sec_max;
      __ts.tv_nsec = 999999999; // (10^9 - 1)
    }
 
    while (nanosleep(&__ts, &__ts) == -1 && errno == EINTR);
 }
 
 // Thread local storage
-int __libcpp_tls_create(__libcpp_tls_key *__key, void (*__at_exit)(void *))
+int __libcpp_tls_create(__libcpp_tls_key *__key,
+                        void (_LIBCPP_TLS_DESTRUCTOR_CC* __at_exit)(void *))
 {
   return pthread_key_create(__key, __at_exit);
 }
 
 void *__libcpp_tls_get(__libcpp_tls_key __key)
 {
   return pthread_getspecific(__key);
 }