golang.org/x/tools/go/analysis: pass "gccgo" to types.SizesFor

For the gofrontend copy, change calls to types.SizesFor to pass
"gccgo" rather than "gc".  Leave the asmdecl pass unchanged since that
pass is gc-specific anyhow.

This has been fixed in a better way in the external repo by
https://golang.org/cl/158317 and friends, but that is not in 1.12, so
use this approach for now.

Change-Id: Ia9d135d88be4544dc51a1a0837d691512b7305aa
Reviewed-on: https://go-review.googlesource.com/c/158842
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
4 files changed