rtld: Remove calculate_tls_end

Variant I architectures use off and Variant II ones use size + off.
Define TLS_VARIANT_I/TLS_VARIANT_II symbols similarly to how libc
handles it.

Reviewed by: kib
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31539
Differential revision: https://reviews.freebsd.org/D31541