no need to mess with KBUILD_CFLAGS on uml-i386 anymore

Now that X86_32 is provided on Kconfig level for uml-i386, there's no
need to play with it explicitly on Makefile level anymore.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>