uml: keep UML Kconfig in sync with x86

Fix a UML build breakage introduced by commit
1032c0ba

 - it introduces X86_32, with many
things which UML needs depending on it.

This patch adds definitions of X86_32 and RWSEM_XCHGADD_ALGORITHM to
the UML/i386 Kconfig.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>