go/build: allow @ in #cgo lines for OS X special paths

[andlabs]

At present, safeCgoName in src/go/build/build.go does not allow @ in a #cgo line, preventing the use of @ on any platform. Simple nonworking test:

package main
// #cgo LDFLAGS: @fail
import "C"

results in

can't load package: /tmp/x.go: malformed #cgo argument: @fail

Why is @ needed? OS X does not use $ORIGIN in its rpath for indicating that binaries should look in the binary's directory for their shared libraries; instead, it uses the special name @executable_path. There are a number of these special names provided by Apple's loader (and more in other links I can't find right now).

I don't know what the consequences of adding @ to the list of safe cgo runes would be, or why @ was omitted, so I am submitting a bug instead of a patch.

After this fix, Go packages that use -rpath @executable_path in its #cgo LDFLAGS line should be able to build. Furthemore, TestCgoHandlesWlORIGIN in src/cmd/go/go_test.go could also be adjusted to work on OS X as well.

I am running

go version devel +49065cb Fri Sep 18 18:09:15 2015 +0000 linux/amd64

but manually confirmed the issue will still happen on master. Here is an issue where someone else runs into it in my code using stable go1.5.2.

[gopherbot]

CL https://golang.org/cl/18331 mentions this issue.