No, '__mempcpy' is used by user-space applications, like coreutils for example and glibc still has '__mempcpy' as a global symbol. How about this, do a complete buildroot file system after hacking uClibc to make sure things work? There's a novel idea.

libc/string/generic/mempcpy.c

@@ -9,7 +9,7 @@
 
 #undef mempcpy
 
-void attribute_hidden *__mempcpy (void *dstpp, const void *srcpp, size_t len)
+void *__mempcpy (void *dstpp, const void *srcpp, size_t len)
 {
   __memcpy(dstpp, srcpp, len);
   return (void *)(((char *)dstpp) + len);