crypto/tls: help linker remove code when only Client or Server is used

This saves 166 KiB for a tls.Dial hello world program (5382441 to
5212356 to bytes), by permitting the linker to remove TLS server code.

Change-Id: I16610b836bb0802b7d84995ff881d79ec03b6a84
Reviewed-on: https://go-review.googlesource.com/c/go/+/228111


Reviewed-by: Ian Lance Taylor <iant@golang.org>