gccgo: internal compiler error in methods

[dvyukov]

gccgo crashes on the following program:

package a
var _ interface{}

go1: internal compiler error: in methods, at go/gofrontend/types.cc:7076
0x851f41 Interface_type::methods() const
    ../../gcc/go/gofrontend/types.cc:7076
0x7a31eb Gogo::build_interface_method_tables()
    ../../gcc/go/gofrontend/gogo.cc:4316
0x7c68f6 Gogo::build_interface_method_tables()
    ../../gcc/go/gofrontend/gogo.cc:1301
0x7c68f6 Gogo::write_globals()
    ../../gcc/go/gofrontend/gogo.cc:1272

gcc version 6.0.0 2015070 (experimental) (GCC)

[dvyukov]

@paranoiacblack

[gopherbot]

CL https://golang.org/cl/12049 mentions this issue.