rtld-elf: Mark tls_init_align __unused in free_tls for Variant I TLS.

Some architectures (powerpc and RISC-V) always use 0 for the post TLS
size in which case tls_init_align isn't used by
calculate_tls_post_size. Use __unused to quiet the warning for these
platforms.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D40673