Add abort () to jerry-libc

Added declaration and implementations of `void abort (void)` to
jerry-libc. As the linux implementation relies on the `getpid`
syscall - which has no arguments - `syscall_0` implementations
have been added as well.

`libc_fatal` has been adapted to use the new `abort` function
instead of `exit`. This also made the definition of
`LIBC_FATAL_ERROR_EXIT_CODE` unnecessary.

Finally, the syscall fatal error message was fixed.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-libc/jerry-libc-fatals.c

@@ -41,5 +41,5 @@ libc_fatal (const char *msg, /**< fatal error description */
             msg, function_name, file_name, line_number);
   }
 
-  exit (LIBC_FATAL_ERROR_EXIT_CODE);
+  abort ();
 } /* libc_fatal */