arm64: bs_sr_<N> take II

In r367327 generic_bs_sr_<n> were derived from mips. Given we are calling
generic_bs_w_<n> and no write directly, we do not have to do the address
calculations ourselves as eneric_bs_w_<n> will do a str val [bsh, offset].
All we actually have to do is increment offset.

MFC after: 3 days