re PR go/79146 (Bootstrapping go on s390x fails; redefined symbols)

	PR go/79146
    math/big: fix build on s390x
    
    Don't build arith_decl_s390x.go for gccgo; it is only for assembly
    code that has not yet been ported to gccgo.
    
    For GCC PR 79146.

From-SVN: r244731

gcc/go/gofrontend/MERGE

-a21ebb50459ba9608d0325ad1c1d62e6c9290186
+907f6e31975443993c47fa45e09cf85d0709b7e6
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/math/big/arith_decl_s390x.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
 // +build !math_big_pure_go
 
 package big