Makefile: Define __ASSEMBLY__ for assembly files

There are 25 header files today (found with grep -r "#ifndef __ASSEMBLY__)
with functionality relies on the __ASSEMBLY__ prepocessor constant being
correctly defined to work correctly. So far, kvm-unit-tests has relied on
the assembly files to define the constant before including any header
files which depend on it.

Let's make sure that nobody gets this wrong and define it as a compiler
constant when compiling assembly files. __ASSEMBLY__ is now defined for all
.S files, even those that didn't set it explicitely before.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>