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

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>