Kernel+LibC: Tidy up assertion failures with a dedicated syscall

This patch adds sys$abort() which immediately crashes the process with
SIGABRT. This makes assertion backtraces a lot nicer by removing all
the gunk that otherwise happens between __assertion_failed() and
actually crashing from the SIGABRT.

Kernel/CMakeLists.txt

@@ -104,6 +104,7 @@ set(KERNEL_SOURCES
     SharedBuffer.cpp
     StdLib.cpp
     Syscall.cpp
+    Syscalls/abort.cpp
     Syscalls/access.cpp
     Syscalls/alarm.cpp
     Syscalls/beep.cpp