misc/cgo/testcshared: add explicit ./ to shared library argument

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>

misc/cgo/testcshared/test.bash

@@ -105,7 +105,7 @@ status=0
 
 # test0: exported symbols in shared lib are accessible.
 # TODO(iant): using _shared here shouldn't really be necessary.
-$(go env CC) ${GOGCCFLAGS} -I ${installdir} -o testp main0.c libgo.$libext
+$(go env CC) ${GOGCCFLAGS} -I ${installdir} -o testp main0.c ./libgo.$libext
 binpush testp
 
 output=$(run LD_LIBRARY_PATH=. ./testp)