x86: Work around mmio config space quirk on AMD Fam10h

Some broken devices have been discovered to require %al/%ax/%eax registers
for MMIO config space accesses.  Modify mmconfig.c to use these registers
explicitly (rather than modify the global readb/writeb/etc inlines).

AK: also changed i386 to always use eax
AK: moved change to extended space probing to different patch
AK: reworked with inlines according to Linus' requirements.
AK: improve comments.

Signed-off-by: dean gaudet <dean@arctic.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>