PCI/MSI: Use #ifdefs instead of weak functions

Weak functions aren't all they're cracked up to be. They lead to
incorrect binaries with some toolchains, they require us to have empty
functions we otherwise wouldn't, and the unused code is not elided
(as of gcc 4.3.2 anyway).

So replace the weak MSI arch hooks with the #define foo foo idiom. We no
longer need empty versions of arch_setup/teardown_msi_irq().

This is less source (by 1 line!), and results in smaller binaries too:

   text	   data	    bss	    dec	    hex	filename
9354300	1693916	 678424	11726640 b2ef30	build/powerpc/vmlinux-before
9354052	1693852	 678424	11726328 b2edf8	build/powerpc/vmlinux-after

Also smaller on x86_64 and arm (iop13xx).

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>