string.h: fix memset over-optimization with clang

We found a weird case of parasite code dying of SIGSEGV when clang
is used as a compiler (see [1] for more details).

Apparently, it was caused by clang optimizing our builtin_memset()
by inserting a call to memset(). It is a valid compiler optimization,
aside from the fact that in our code memset() is defined as a weak
alias to builtin_memset(), which of course lead to infinite recursion
and stack growth.

This might be a bug in compiler, but there are ways to avoid it:

1. Rewrite builtin_memset() in asm (note it needs to be done
   for every architecture supported).
2. Disable compiler optimizations for this code (say, by using -O0).
3. Declare the pointer inside builtin_memcpy() as volatile.

The last approach looks more appealing -- mostly for being simple.

[1] https://github.com/xemul/criu/issues/279

travis-ci: success for string.h: fix memset over-optimization with clang
Cc: Andrei Vagin <avagin@virtuozzo.com>
Cc: Dmitry Safonov <dsafonov@virtuozzo.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/include/asm-generic/string.h

@@ -65,7 +65,7 @@ static always_inline int builtin_strncmp(const char *cs, const char *ct, size_t
 #ifndef HAS_BUILTIN_MEMSET
 static __maybe_unused void *builtin_memset(void *s, const int c, size_t count)
 {
-	char *dest = s;
+	volatile char *dest = s;
 	size_t i = 0;
 
 	while (i < count)