um: most of the SUBARCH uses can be killed

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[richard@nod.at: Re-export SUBARCH in arch/um/Makefile]
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/Makefile

@@ -3,7 +3,7 @@
 # Licensed under the GPL
 #
 
-CPPFLAGS_vmlinux.lds := -U$(SUBARCH) -DSTART=$(LDS_START) \
+CPPFLAGS_vmlinux.lds := -DSTART=$(LDS_START) \
                         -DELF_ARCH=$(LDS_ELF_ARCH)        \
                         -DELF_FORMAT=$(LDS_ELF_FORMAT)
 extra-y := vmlinux.lds