Enable --enable-fortify-source with clang

clang generates internal calls for some _chk symbol, so add internal
aliases for them, and stub some with rtld-stubbed-symbols to avoid
ld.so linker issues.

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

string/Makefile

@@ -121,6 +121,7 @@ routines := \
 
 # Exclude fortified routines from being built with _FORTIFY_SOURCE
 routines_no_fortify += \
+  bzero \
   explicit_bzero \
   memcpy \
   memmove \