go/printer, gofmt: avoid extra final comma in multi-line signatures

go/printer, gofmt: avoid extra final comma in multi-line signatures

The parameter list layout function was incorrectly computing the
end of the previous line in cases where a parameter type spanned
multiple lines. As a result, an extra (valid, but not needed)
comma was introduced before the paremeter list's closing parenthesis.

Fixes #4533.

R=rsc
CC=golang-dev
https://golang.org/cl/7674044