[PATCH] uml: tidy biarch gcc support

On top of the previous biarch changes for UML, this makes the preprocessor
changes a bit cleaner.  Specify the 64-bit build in CPPFLAGS on the x86_64
SUBARCH, rather than #undef'ing i386.  Compile-tested with i386 and x86_64
SUBARCHs.

Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>