cmd: generate DWARF for functions in compile instead of link.

This is a copy of golang.org/cl/22092 by Ryan Brown.

Here's his original comment:
On my machine this increases the average time for 'go build cmd/go' from
2.25s to 2.36s. I tried to measure compile and link separately but saw
no significant change.

Change-Id: If0d2b756d52a0d30d4eda526929c82794d89dd7b
Reviewed-on: https://go-review.googlesource.com/25311
Run-TryBot: Michael Matloob <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: David Crawshaw <[email protected]>

Author: matloob

src/cmd/dist/buildtool.go

@@ -41,6 +41,7 @@ var bootstrapDirs = []string{
 	"compile/internal/s390x",
 	"internal/bio",
 	"internal/gcprog",
+	"internal/dwarf",
 	"internal/obj",
 	"internal/obj/arm",
 	"internal/obj/arm64",