godoc/dl: update some pretty GOARCH strings

Fixes golang/go#19082

Change-Id: Id0e985eeff543ad24ff969ff8c94e086a7bc9303
Reviewed-on: https://go-review.googlesource.com/46916
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
diff --git a/godoc/dl/dl.go b/godoc/dl/dl.go
index 7c1b15c..abff327 100644
--- a/godoc/dl/dl.go
+++ b/godoc/dl/dl.go
@@ -491,10 +491,10 @@
 	"linux":   "Linux",
 	"windows": "Windows",
 
-	"386":   "32-bit",
-	"amd64": "64-bit",
-
+	"386":    "x86",
+	"amd64":  "x86-64",
 	"armv6l": "ARMv6",
+	"arm64":  "ARMv8",
 
 	"archive":   "Archive",
 	"installer": "Installer",