ARM: assembler.h: Add string declaration macro

Declaring strings in assembler source involves a certain amount of
tedious boilerplate code in order to annotate the resulting symbol
correctly.

Encapsulating this boilerplate in a macro should help to avoid some
duplication and the occasional mistake.

Signed-off-by: Dave Martin <dave.martin@linaro.org>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>