package/go: fix GO_ARCH for s390x arch

Fixes:
- http://autobuild.buildroot.net/results/82c440825290804c2fd6a92d916c93d2934e0a95
- http://autobuild.buildroot.net/results/e8f8e232fb6bf6167f25c82026f4e2321cca9c26
- http://autobuild.buildroot.net/results/7d4137c439ade81ab7bf141ad6504a69746d6dbc

Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/go/go.mk

@@ -54,6 +54,8 @@ else ifeq ($(BR2_mips64),y)
 GO_GOARCH = mips64
 else ifeq ($(BR2_mips64el),y)
 GO_GOARCH = mips64le
+else ifeq ($(BR2_s390x),y)
+GO_GOARCH = s390x
 endif
 
 # For the convienience of target packages.