[kernel] Change kernel log levels

The kernel log levels are now numerically reversed so that more-verbose
levels can be added to the end. Replaced 'debug' with 'verbose', and
added new 'spam' level.

src/kernel/syscalls/system.cpp

@@ -32,7 +32,7 @@ j6_status_t
 noop()
 {
     thread &th = thread::current();
-    log::debug(logs::syscall, "Thread %llx called noop syscall.", th.koid());
+    log::verbose(logs::syscall, "Thread %llx called noop syscall.", th.koid());
     return j6_status_ok;
 }