x86: unify BUG() & Co, reduce overhead on x86-64

x86: unify BUG() & Co, reduce overhead on x86-64

Since it's only the string pointer representations that differ between
i386 and x86-64, abstract out those and make everything else shared.

While touching this code, also use
- proper instructions rather than a mixture of such and raw .byte/
  .long/.quad data emissions,
- PC-relative pointers on x86-64 to cut the amount of storage (and
  in particular cache space) needed for string references by half.

Signed-off-by: Jan Beulich <jbeulich@novell.com>