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.

Libraries/LibC/assert.cpp

@@ -47,8 +47,8 @@ void __assertion_failed(const char* msg)
         { msg, strlen(msg) },
     };
     syscall(SC_set_coredump_metadata, &params);
-
-    abort();
+    syscall(SC_abort);
+    for (;;) { }
 }
 #endif
 }