This allows passing custom LDFLAGS while building the bootstrapping
tool.

Afterwards, GO_LDFLAGS will be used as usual.

Change-Id: I1e224e3ce8bf7b2ce1ef8fec1894720338f04396
GitHub-Last-Rev: 17d40dc2dd2f0815331cb2f8de3445f86687cc45
GitHub-Pull-Request: golang/go#31298
Reviewed-on: https://go-review.googlesource.com/c/go/+/171037
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>