elf: Fix tunable handing with clang

Recent clang version optimizes some loops contructions to strlen [1],
which might generate function calls when self-relocation is not
already done (on tunable parsing).  Use an out-of-line function
with __attribute_optimization_barrier__ to avoid this.

[1] facd7dfc80

Reviewed-by: Sam James <sam@gentoo.org>

elf/dl-printf.c

@@ -35,6 +35,7 @@
 
 /* The function might be called before the process is self-relocated.  */
 static size_t
+__attribute_optimization_barrier__
 _dl_debug_strlen (const char *s)
 {
   const char *p = s;