ssp: add APIs for Stack Smashing Protection

Compiling with any of the -fstack-protector* flags requires the
__stack_chk_guard data import (which needs to be initialized) and the
__stack_chk_fail{,_local} functions.  While GCC's own libssp can provide
these, it is better that we provide these ourselves.  The implementation
is custom due to being OS-specific.

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>

newlib/libc/ssp/stack_protector.c

@@ -0,0 +1,45 @@
+#include <sys/cdefs.h>
+#include <sys/param.h>
+#include <signal.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+uintptr_t __stack_chk_guard = 0;
+
+void
+__attribute__((__constructor__))
+__stack_chk_init (void)
+{
+  if (__stack_chk_guard != 0)
+    return;
+
+#if defined(__CYGWIN__) || defined(__rtems__)
+  arc4random_buf(&__stack_chk_guard, sizeof(__stack_chk_guard));
+#else
+  /* If getentropy is not available, use the "terminator canary". */
+  ((unsigned char *)&__stack_chk_guard)[0] = 0;
+  ((unsigned char *)&__stack_chk_guard)[1] = 0;
+  ((unsigned char *)&__stack_chk_guard)[2] = '\n';
+  ((unsigned char *)&__stack_chk_guard)[3] = 255;
+#endif
+}
+
+void
+__attribute__((__noreturn__))
+__stack_chk_fail (void)
+{
+  char msg[] = "*** stack smashing detected ***: terminated\n";
+  write (2, msg, strlen (msg));
+  raise (SIGABRT);
+  _exit (127);
+}
+
+#ifdef __ELF__
+void
+__attribute__((visibility ("hidden")))
+__stack_chk_fail_local (void)
+{
+	__stack_chk_fail();
+}
+#endif