[dev.link] cmd/link: use the start of compilation unit for R_ADDRCUOFF

[dev.link] cmd/link: use the start of compilation unit for R_ADDRCUOFF

R_ADDRCUOFF refers to the offset from the start of the
compilation unit. Now that we could have multiple compilation
units per package, we should use the start of the compilation
unit, instead of the start of the package.

Change-Id: I3d6a3c5a5c47c9ccf88091ed05413ced826f9934
Reviewed-on: https://go-review.googlesource.com/c/go/+/204828
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>