cmd/go, misc/cgo: skip test if no .edata

Clang does not produce binaries with an .edata section, even when it
exports symbols properly, so just skip this binutils-specific test for
that case. Later we can rewrite these tests entirely to do something
more robust.

Updates #46719.

Change-Id: I864b3c2d91e66800c55454ae11d4ab1623693d14
Reviewed-on: https://go-review.googlesource.com/c/go/+/327549


Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>