cmd/internal/obj: combine Pcrel and Comefrom

cmd/internal/obj: combine Pcrel and Comefrom

They're each architecture-specific.
Let them share.

Reduces Prog size to 288, which is the
next smaller malloc class.

Reduces inuse_space while compiling the
rotate tests by ~3.2%.

Change-Id: Ica8ec90e466c97b569745fffff0e5acd364e55fa
Reviewed-on: https://go-review.googlesource.com/10514
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>