Booted into 64-bit long mode.

Some lessons learned so far:
- The 510-bytes limitation of MBR was quite painful.
- Without '-n' or '-N' option, x86_64-ld seems to align the program
  header section to 2M page boundary regardless the ALIGN directive. It
  may not be desirable.
- The 64-bit GDT. The trick part is its descriptor, the GDTR. Although
  its 32-bit counterpart has the format word + long, one would expect
  itself to be long + quad. It is not. The AMD64 manual volume 2 section
  4.6.2 has all the details. A little bit counter-intuitive and very
  hard to debug if gets wrong.