cmd/go: -buildmode=shared and cgo and -compiler gccgo don't work together #15885
Description
This combination has probably never worked correctly, but my fix for #11739 broke it harder:
$ cat $GOPATH/src/pkgconfigcgo/p.go
package pkgconfigcgo
// #cgo pkg-config: libpng
// #include <png.h>
import "C"
func V() uint32 {
return uint32(C.png_access_version_number())
}
$ go build -buildmode=shared -compiler gccgo pkgconfigcgo
# /tmp/go-build393723771/libpkgconfigcgo.so
/opt/opensource/gopath/pkg/gccgo_linux_amd64_fPIC/libpkgconfigcgo.a: member /opt/opensource/gopath/pkg/gccgo_linux_amd64_fPIC/libpkgconfigcgo.a(_cgo_flags) in archive is not an object
collect2: error: ld returned 1 exit status
gccgo's ldShared() needs to do (at least) some of the tricks its ld() method does wrt _cgo_flags and so on.
No rush for 1.7 – it's more important to me to get a fix into gccgo in Ubuntu.