FORTIFY_SOURCE: strlen check.

This test is designed to detect code such as:

int main() {
  char buf[10];
  memcpy(buf, "1234567890", sizeof(buf));
  size_t len = strlen(buf); // segfault here with _FORTIFY_SOURCE
  printf("%d\n", len);
  return 0;
}

or anytime strlen reads beyond an object boundary. This should
help address memory leakage vulnerabilities and make other
unrelated vulnerabilities harder to exploit.

Change-Id: I354b425be7bef4713c85f6bab0e9738445e00182

libc/Android.mk

@@ -217,6 +217,7 @@ libc_common_src_files := \
 	string/__strcpy_chk.c \
 	string/__strlcat_chk.c \
 	string/__strlcpy_chk.c \
+	string/__strlen_chk.c \
 	string/__strncat_chk.c \
 	string/__strncpy_chk.c \
 	wchar/wcpcpy.c \