x86-64: clean up io-apic accesses

This is just commit 130fe05dbc0114609cfef9815c0c5580b42decfa ported to
x86-64, for all the same reasons.  It cleans up the IO-APIC accesses in
order to then fix the ordering issues.

We move the accessor functions (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>