compiler.h clean up

[Matt Turner]

This is the first round of patches to clean up that awful piece of code known
as compiler.h.

The first six patches organize and fix the memory barrier macros. The last
three patches remove dead code. They should be pretty self-explanatory.

Please review and commit. I'm going to make compiler.h fun to look at.

 hw/xfree86/common/compiler.h |  302 ++++++++++++++----------------------------
  1 files changed, 102 insertions(+), 200 deletions(-)