[x86] C1 StubAssembler::call_RT, "call_offset might not be initialized"

Static analyzers complain `call_offset` might not be initialized in `StubAssembler::call_RT` in `c1_Runtime1_x86.cpp`. The way I see it, it depends on `align_stack` value, and it is set whether it `align_stack` is `true` or `false`. But we can probably make it cleaner so that future errors would be clearly detectable.