disable _FORTIFY_SOURCE under clang

Clang and _FORTIFY_SOURCE are just plain incompatible with
each other.  First of all, clang doesn't understand the
__attribute__((gnu_inline)) header. Second of all,
Clang doesn't have support for __builtin_va_arg_pack()
and __builtin_va_arg_pack_len() (see
http://clang.llvm.org/docs/UsersManual.html#c_unimpl_gcc)

Until we can resolve these issues, don't even try using
_FORTIFY_SOURCE under clang.

Change-Id: I81c2b8073bb3276fa9a4a6b93c427b641038356a

libc/include/string.h

@@ -147,7 +147,6 @@ void *memset (void *s, int c, size_t n) {
     return __builtin___memset_chk(s, c, n, __builtin_object_size (s, 0));
 }
 
-#if !defined(__clang__)
 extern size_t __strlcpy_real(char *, const char *, size_t)
     __asm__(__USER_LABEL_PREFIX__ "strlcpy");
 extern void __strlcpy_error()
@@ -224,8 +223,6 @@ size_t strlen(const char *s) {
 
     return __strlen_chk(s, bos);
 }
-#endif  /* !defined(__clang__) */
-
 
 #endif /* defined(__BIONIC_FORTIFY_INLINE) */