cmd/go: go vet should tell cmd/vet the names of .s files

[alandonovan]

$ go version
go version devel +c5d38b896d Thu Aug 23 06:10:48 2018 +0000 linux/amd64

$ go list -f {{.SFiles}} math/big
[arith_amd64.s]

$ go vet -x math/big
...
cat >$WORK/b001/vet.cfg << 'EOF' # internal
{
        "Dir": "/home/adonovan/go/src/math/big",
        "ImportPath": "math/big",
        "GoFiles": [
                "/home/adonovan/go/src/math/big/accuracy_string.go",
                ...,
        ],
        ...,
}
EOF
...

Some vet checks, such as asmdecl and buildtag, apply equally to the Go files and non-Go files of a package. However, the .cfg file that go vet passes to cmd/vet does not mention them, so these checks are incomplete or ineffective.

The .cfg file should contain a field NonGoFiles that lists the package source files written in assembly, C, Fortran, and all the other benighted languages.

[gopherbot]

Change https://golang.org/cl/148904 mentions this issue: cmd/go: vet: pass non-.go files to vet tool