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>

tests/checkasm/arm/checkasm.S

@@ -35,9 +35,9 @@ endconst
 
 const error_message_fpscr
     .asciz "failed to preserve register FPSCR"
-const error_message_gpr
+error_message_gpr:
     .asciz "failed to preserve register r%d"
-const error_message_vfp
+error_message_vfp:
     .asciz "failed to preserve register d%d"
 endconst