[x86_64] Fix assorted 64-bit compilation errors and warnings

Remove various 32-bit assumptions scattered throughout the codebase.
The code is still not necessarily 64-bit clean, but will at least
compile.

src/drivers/infiniband/arbel.c

@@ -27,6 +27,7 @@
 #include <unistd.h>
 #include <errno.h>
 #include <byteswap.h>
+#include <gpxe/io.h>
 #include <gpxe/pci.h>
 #include <gpxe/malloc.h>
 #include <gpxe/umalloc.h>