runtime: convert _func.entry to a method

runtime: convert _func.entry to a method

A subsequent change will alter the semantics of _func.entry.
To make that change obvious and clear, change _func.entry to a method,
and rename the field to _func.entryPC.

Change-Id: I05d66b54d06c5956d4537b0729ddf4290c3e2635
Reviewed-on: https://go-review.googlesource.com/c/go/+/351460
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>