checkasm: arm: Use X() instead of inline ifdefs

This works fine when the referenced symbol has the same prefix
as PRIVATE_PREFIX in the same file; otherwise we could also
create a macro like X() that only prepends the extern symbol
prefix but no symbol namespace prefix.

tests/checkasm/arm/checkasm_32.S

@@ -185,11 +185,7 @@ function checked_call_\variant, export=1
 2:
         movrel          r0, error_message_gpr
 1:
-#ifdef PREFIX
-        bl              _checkasm_fail_func
-#else
-        bl              checkasm_fail_func
-#endif
+        bl              X(fail_func)
 0:
         pop             {r0, r1}
 .ifc \variant, vfp

tests/checkasm/arm/checkasm_64.S

@@ -190,11 +190,7 @@ function checked_call, export=1
 2:
         movrel          x0, error_message_stack
 1:
-#ifdef PREFIX
-        bl              _checkasm_fail_func
-#else
-        bl              checkasm_fail_func
-#endif
+        bl              X(fail_func)
 0:
         ldp             x0,  x1,  [sp], #16
         ldp             d14, d15, [sp], #16