cmd/link: disable TestPIESize if CGO isn't enabled

With CGO disabled, the test throws the following error:

elf_test.go:291: # command-line-arguments
    loadinternal: cannot find runtime/cgo

Change-Id: Iaeb183562ab637c714240b49e73078bdb791b35b
GitHub-Last-Rev: f8fe9afad5611411966413d17cb5874f7b0018a0
GitHub-Pull-Request: golang/go#43911
Reviewed-on: https://go-review.googlesource.com/c/go/+/286632


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>