toolchain: Add assembly-side counterpart of __aligned

Zephyr toolchain abstraction provides a macro __aligned(x) to add specified
alignment, in bytes, to a chosen symbol for C/C++ source files but there is
no portable counterpart available for symbols defined in assembly source
files. This change-set adds a new macro ALIGN(x) for this purpose.

Signed-off-by: Irfan Ahmad <irfan.ahmad@siemens.com>