checkasm: arm: Don't start new const blocks for each string

Each const block needs to be terminated by one endconst
invocation so either call endconst after each, or just
declare plain labels to the later strings.

This fixes errors such as this, on some binutils versions:

checkasm.S:38: Error: Macro `endconst' was already defined

Signed-off-by: Martin Storsjö <martin@martin.st>
