rtld-elf: Push TLS_DTV_OFFSET into tls_get_addr_common's arguments

[jrtc27]

Summary

Rather than calling tls_get_addr_common with a biased ti_offset and
un-biasing the return value, we can instead un-bias the offset given in
the first place and not need to adjust the return value. This is in fact
how the MIPS implementation worked, and makes this call tail-recursive.