build: run targets csopes,tags,.. without Rules.mk

build: run targets csopes,tags,.. without Rules.mk

Those targets make use of $(all_sources) which depends on TARGET_ARCH,
so we just need to set TARGET_ARCH earlier and once.

XEN_TARGET_ARCH isn't expected to change during the build, so
TARGET_SUBARCH and TARGET_ARCH aren't going to change either. Set them
once and for all in the Xen root Makefile. This allows to run more
targets without Rules.mk.

XEN_TARGET_ARCH is actually changed in arch/x86/boot/build32.mk, but
it doesn't use the TARGET_{,SUB}ARCH variables either, and doesn't use
Rules.mk (it replaces it).

TARGET_{,SUB}ARCH are no longer overridden because that would have
no effect on the values that Rules.mk will use.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>