runtime: mark concatstring as variadic (fixes 386 build)

runtime: mark concatstring as variadic (fixes 386 build)

Windows was the only one seeing this bug reliably in the builder,
but it was easy to reproduce using 'GOGC=1 go test strconv'.
concatstring looked like it took only one string, but in fact it
takes a long list of strings. Add an explicit ... so that the traceback
will not use the "fixed" frame size and instead look at the
frame size metadata recorded by the caller.

R=golang-dev
TBR=golang-dev
CC=golang-dev
https://golang.org/cl/11531043