i386: clean up io-apic accesses

This is preparation for fixing the ordering of the accesses that
got broken by the commit cf4c6a2f

 when
factoring out the "common" io apic routing entry accesses.

Move the accessor function (that were only used by io_apic.c) out
of a header file, and use proper memory-mapped accesses rather than
making up our own "volatile" pointers.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>