build: avoid building arm/arm/*/head.o twice

build: avoid building arm/arm/*/head.o twice

head.o is been built twice, once because it is in $(ALL_OBJS) and a
second time because it is in $(extra-y) and thus it is rebuilt when
building "arch/arm/built_in.o".

Fix this by adding a dependency of "head.o" on the directory
"arch/arm/".

Also, we should avoid building object that are in subdirectories, so
move the declaration in there. This doesn't change anything as
"arch/arm/built_in.o" depends on "arch/arm/$subarch/built_in.o" which
depends on $(extra-y), so we still need to depend on
"arch/arm/built_in.o".

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>