cmd/go: find GOROOT using os.Executable when installed to GOROOT/bin/GOOS_GOARCH

cmd/go: find GOROOT using os.Executable when installed to GOROOT/bin/GOOS_GOARCH

When running make.bash in a cross-compiled configuration
(for example, GOARCH different from GOHOSTARCH), cmd/go
is installed to GOROOT/bin/GOOS_GOARCH instead of GOROOT/bin.

That means that we need to look for GOROOT in both ../.. and ../../..,
not just the former.

Fixes #62119.
Updates #18678.

Change-Id: I283c6a10c46df573ff44da826f870417359226a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/521015
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>