tls_start_address/tls_start_address_aarch64: refactoring

[marxin]

Apparently, for the RISC-V port, I am going to introduce yet another version of the function, and I think the function should be generic over A:Arch, and we can get rid of the special handling for various archs: RelocationKind::TpOffAArch64.

[marxin]

Fixed as part of #877.