Update pthread_self to stay in sync with upstream architectures:

Change references this commit:
commit 7284830
Author: Rich Felker <dalias@aerifal.cx>
Date:   Mon Aug 24 22:23:08 2020 -0400

    deduplicate __pthread_self thread pointer adjustment out of each arch

arch/hexagon/pthread_arch.h

@@ -1,8 +1,7 @@
-
 // Hexagon supports variant 2 TLS.
-static inline struct pthread *__pthread_self()
+static inline uintptr_t __get_tp()
 {
-  struct pthread *tp;
+  uintptr_t tp;
   __asm__ ( "%0 = ugp" : "=r"(tp));
   return tp;
 }