ml: fix compilation options for USER_OBJS

From: Jeff Dike <jdike@addtoit.com>, Paolo Giarrusso <blaisorblade@yahoo.it>

Make sure that when compiling USER_OBJS the correct compilation options are
passed; since they are compiled with USER_CFLAGS which is derived from
CFLAGS, make sure it is a recursively evaluated variable, so that changes
to CFLAGS done afterwards the inclusion of arch/$(ARCH)/Makefile are
reflected in USER_CFLAGS.

For instance, without this patch userspace objects are never compiled with
debug info active.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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