Use an explicit ./ to make sure we link against the libgo.so we just
built, not some other libgo.so that the compiler or linker may decide to
seek out.

Fixes #17986.

Change-Id: Id23f6c95aa2b52f4f42c1b6dac45482c22b4290d
Reviewed-on: https://go-review.googlesource.com/33413
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>