deduplicate TP_ADJ logic out of each arch, replace with TP_OFFSET

the only part of TP_ADJ that was not uniquely determined by
TLS_ABOVE_TP was the 0x7000 adjustment used mainly on mips and powerpc
variants.

src/internal/pthread_impl.h

@@ -105,10 +105,20 @@ struct __timer {
 #define CANARY canary
 #endif
 
+#ifndef TP_OFFSET
+#define TP_OFFSET 0
+#endif
+
 #ifndef DTP_OFFSET
 #define DTP_OFFSET 0
 #endif
 
+#ifdef TLS_ABOVE_TP
+#define TP_ADJ(p) ((char *)(p) + sizeof(struct pthread) + TP_OFFSET)
+#else
+#define TP_ADJ(p) (p)
+#endif
+
 #ifndef tls_mod_off_t
 #define tls_mod_off_t size_t
 #endif