libc: add __explicit_bzero_chk()

Needed by newer version ruby.

Signed-off-by: KANATSU Minoru <icc.po...@gmail.com>
Message-Id: <20190528135740.32121-1-icc.pot.tyew272@gmail.com>

Makefile

@@ -1568,6 +1568,7 @@ libc += stdlib/strtod.o
 libc += stdlib/wcstol.o
 
 libc += string/__memcpy_chk.o
+libc += string/__explicit_bzero_chk.o
 musl += string/bcmp.o
 musl += string/bcopy.o
 musl += string/bzero.o

libc/string/__explicit_bzero_chk.c

@@ -0,0 +1,11 @@
+#include <string.h>
+#include <stdlib.h>
+
+void __explicit_bzero_chk(void *dest, size_t len, size_t destlen)
+{
+    if (len > destlen) {
+        abort();
+    }
+    memset(dest, 0, len);
+    return;
+}