libc: Reassociate pointer arithmetic in __libc_tls_get_addr

Rather than compute a biased pointer only to then un-bias it again, un-bias
the offset before adding it to the DTV entry.

This mirrors rtld-elf commit d71c97026366 ("rtld-elf: Push
TLS_DTV_OFFSET into tls_get_addr_common's arguments")

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D50224