cmd/link: make funcSize a constant

cmd/link: make funcSize a constant

Now that it no longer depends on the size of a pointer,
we can make it a constant, which simplifies a bit of code.

Change-Id: I1b7c3b1b648da5c8960378a02b9263e2cc902441
Reviewed-on: https://go-review.googlesource.com/c/go/+/352952
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>