[panic] Add separate kernel-mode panic handler

Created the framework for using different loadable panic handlers,
loaded by the bootloader. Initial panic handler is panic.serial, which
contains its own serial driver and stacktrace code.

Other related changes:

- Asserts are now based on the NMI handler - panic handlers get
  installed as the NMI interrupt handler
- Changed symbol table generation: now use nm's own demangling and
  sorting, and include symbol size in the table
- Move the linker script argument out of the kernel target, and into the
  kernel's specific module, so that other programs (ie, panic handlers)
  can use the kernel target as well
- Some asm changes to boot.s to help GDB see stack frames - but this
  might not actually be all that useful
- Renamed user_rsp to just rsp in cpu_state - everything in there is
  describing the 'user' state

src/kernel/boot.s

@@ -17,16 +17,18 @@ _kernel_header:
 section .text
 align 16
 global _kernel_start:function (_kernel_start.end - _kernel_start)
+global _kernel_start.real
 _kernel_start:
+	push rbp     ; Never executed, fake function prelude
+	mov rbp, rsp ; to calm down gdb
+
+.real:
 	cli
 
 	mov rsp, idle_stack_end
-	mov qword [rsp + 0x00], 0 ; signal end of stack with 0 return address
-	mov qword [rsp + 0x08], 0 ; and a few extra entries in case of stack
-	mov qword [rsp + 0x10], 0 ; problems
-	mov qword [rsp + 0x18], 0
-
+	sub rsp, 16
 	mov rbp, rsp
+
 	extern kernel_main
 	call kernel_main
 
@@ -54,4 +56,3 @@ idle_stack_begin:
 
 global idle_stack_end
 idle_stack_end:
-	resq 4