Fix i386 linux module after r367395.

In r367395 parts of machine dependent linux_dummy.c were moved to a new
machine independent file sys/compat/linux/linux_dummy.c and the existing
linux_dummy.c was renamed to linux_dummy_machdep.c.

Add linux_dummy_machdep.c to the linux module for i386.
Rename sys/amd64/linux32/linux_dummy.c for consistency.
Add the new linux_dummy.c to the linux module for i386.