internal/buildcfg: initialize GOROOT to runtime.GOROOT

In the beginning the Go compiler was in C, and C had a function
'getgoroot' that returned GOROOT from either the environment or a
generated constant. 'getgoroot' was mechanically converted to Go
(as obj.Getgoroot) in CL 3046.

obj.Getgoroot begat obj.GOROOT. obj.GOROOT begat objabi.GOROOT,
which begat buildcfg.GOROOT.

As far as I can tell, today's buildcfg.GOROOT is functionally
identical to runtime.GOROOT(). Let's reduce some complexity by
defining it in those terms.

While we're thinking about buildcfg.GOROOT, also check whether it is
non-empty: if the toolchain is built with -trimpath, the value of
GOROOT might not be valid or meaningful if the user invokes
cmd/compile or cmd/link directly, or via a build tool other than
cmd/go that doesn't care as much about GOROOT. (As of CL 390024,
runtime.GOROOT will return the empty string instead of a bogus one
when built with -trimpath.)

For #51461.

Change-Id: I9fec020d5fa65d4aff0dd39b805f5ca93f86c36e
Reviewed-on: https://go-review.googlesource.com/c/go/+/393155
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/compile/internal/noder/reader.go

@@ -209,7 +209,7 @@ func (pr *pkgReader) posBaseIdx(idx int) *src.PosBase {
 	// require being more consistent about when we use native vs UNIX
 	// file paths.
 	const dollarGOROOT = "$GOROOT"
-	if strings.HasPrefix(filename, dollarGOROOT) {
+	if buildcfg.GOROOT != "" && strings.HasPrefix(filename, dollarGOROOT) {
 		filename = buildcfg.GOROOT + filename[len(dollarGOROOT):]
 	}