cmd/compile/internal/gc: unexport {J,S,F,H,B,V}conv

cmd/compile/internal/gc: unexport {J,S,F,H,B,V}conv

Updates #15462

Unexport Jconv, Sconv, Fconv, Hconv, Bconv, and VConv as they are
not referenced outside internal/gc.

Econv was only called by EType.String, so merge it into that method.

Change-Id: Iad9b06078eb513b85a03a43cd9eb9366477643d1
Reviewed-on: https://go-review.googlesource.com/22531
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>