kernel/x86_64: Do not print a message upon every segmentation fault.

kernel/x86_64: Do not print a message upon every segmentation fault.

It was printed even when the SIGSEGV was caught, such as by configure
tests, causing a rather noisy console when packages were built. After
this commit we're back to the traditional behavior (no message if the
signal is caught, and the usual message if not):

pid <pid> (<user>), uid <uid>: exited on signal 11 (core dumped)

While here, adjust some comments.