checkasm: arm: Don't use blx to call checkasm_fail_func

We should just use a normal bl here, and the linker will add the 'x'
bit if necessary.

This fixes calling the checkasm_fail_func on windows, where the
code is built in thumb mode (and the linker doesn't clear the 'x'
bit in the blx instruction).
Signed-off-by: Martin Storsjö <martin@martin.st>

tests/checkasm/arm/checkasm.S

@@ -150,7 +150,7 @@ function checkasm_checked_call_\variant, export=1
 2:
         movrel          r0, error_message_gpr
 1:
-        blx             X(checkasm_fail_func)
+        bl              X(checkasm_fail_func)
 0:
         pop             {r0, r1}
 .ifc \variant, vfp