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

PR go/79146
    crypto/elliptic: explicitly ignore p256_s390x.go
    
    The file only works if used in conjunction with assembly code not
    (yet) written for gccgo.
    
    Fixes GCC PR 79146.

    Reviewed-on: https://go-review.googlesource.com/35477

From-SVN: r244679

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-2b3d389f961b8461b3fdf42318a628f68b56f8b1
+a21ebb50459ba9608d0325ad1c1d62e6c9290186
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/crypto/elliptic/p256_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 s390x
 
 package elliptic