gorename -from '"cmd/compile/internal/gc".Field.Width' -to Offset

Passes toolstash -cmp.

Change-Id: I310538a1f60bbab470a6375e813e9d5eb52c5bbf
Reviewed-on: https://go-review.googlesource.com/21230Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>