The gc compiler has started permitting go:linkname comments with a
single argument to mean that a function should be externally visible
outside the package. Implement this in the Go frontend.
Change the libgo runtime package to use it, rather than repeating the
name just to export a function.
Remove a couple of unnecessary go:linkname comments on declarations.
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/192197
From-SVN: r275239
Avoids problems with arm64 ILP32 mode. We might want to handle that
mode better in general, but always building panic32.go is simple and
fixes the build.
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/192723
From-SVN: r275237
