Rewrite and hide longjmperror.

This shouldn't be public API, isn't supported on x86/x86_64, and it's
unlikely anyone would have actually seen the message before anyway.
Using __libc_fatal makes it much more likely to be seen.

Bug: 11156955
Change-Id: Icf7f654b22a7dacd89668b60c11e5705c7215c08

libc/bionic/assert.cpp

@@ -34,10 +34,12 @@
 
 void __assert(const char* file, int line, const char* failed_expression) {
   __libc_fatal("%s:%d: assertion \"%s\" failed", file, line, failed_expression);
-  /* NOTREACHED */
 }
 
 void __assert2(const char* file, int line, const char* function, const char* failed_expression) {
   __libc_fatal("%s:%d: %s: assertion \"%s\" failed", file, line, function, failed_expression);
-  /* NOTREACHED */
+}
+
+extern "C" __LIBC_HIDDEN__ void longjmperror() {
+  __libc_fatal("longjmp botch");
 }