[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/panic.serial/serial.h

@@ -0,0 +1,23 @@
+#pragma once
+/// \file panic_serial.h
+/// Non-interrupt-driven serial 'driver' for panic handling
+#include <stdint.h>
+
+namespace panic {
+
+class serial_port
+{
+public:
+	/// Constructor.
+	/// \arg port   The IO address of the serial port
+	serial_port(uint16_t port);
+
+	void write(const char *s);
+
+private:
+	uint16_t m_port;
+};
+
+constexpr uint16_t COM1 = 0x03f8;
+
+} // namespace panic