Recognize gccgo error messages. This uses GCCGO_ERROR, which
is recognized by the gccgo testsuite but is ignored by the
errchk script used with 6g.
method1.go:7:1: error: redefinition of 'M': parameter types changed
method1.go:6:1: note: previous definition of 'M' was here
method1.go:10:1: error: redefinition of 'f': parameter types changed
method1.go:9:1: note: previous definition of 'f' was here
method1.go:13:1: error: redefinition of 'g': parameter names changed
method1.go:12:1: note: previous definition of 'g' was here