[kernel] Add RAII profiler object

Added profiler.h which defines classes and macros for defining profiler
objects. Also added gdb command j6prof for printing profile data. Added
the syscall_profiles profiler class and auto wrapping of syscalls with
profile objects.

Other changes in this commit:

- Made the gdb command `j6threads` argument for specifying a CPU
  optional. Without an argument, it loops through all CPUs.
- Switched to -mcmodel=kernel for kernel code, which makes `call`
  instructions easier to follow when debugging / looking at disassembly.

configs/kernel-debug.yaml

@@ -19,7 +19,7 @@ variables:
     "-mno-sse",
     "-fno-omit-frame-pointer",
     "-mno-red-zone",
-    "-mcmodel=large",
+    "-mcmodel=kernel",
 
     "-g3",
     "-ggdb",