The library initialization code in go-libmain.c sets the C variable
runtime_isarchive but failed to set the Go variable runtime.isarchive.
We don't currently have a way to let C code access an unexported Go
variable, but fortunately the only time the Go function initsig is
called with an argument of true is exactly where we want to set
isarchive. So let initsig do it.
Reviewed-on: https://go-review.googlesource.com/33753
From-SVN: r243094
6e7f1be7ae