x86/um: reuse asm-generic/barrier.h

On x86/um CONFIG_SMP is never defined.  As a result, several macros
match the asm-generic variant exactly. Drop the local definitions and
pull in asm-generic/barrier.h instead.

This is in preparation to refactoring this code area.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Richard Weinberger <richard@nod.at>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>