ipv4: fix multicast socket options on freebsd/arm
ipv4: fix multicast socket options on freebsd/arm

The ARM EABI requires 8-byte stack alignment.

Change-Id: Ibbfbce6aa9ff998dfd6f56217e323045a7057913
Reviewed-on: https://go-review.googlesource.com/11393
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

raboof/net,w4ngyi/net,albertjin/golang-net,muzining/net,kiip/net,yangxianjun/net,DramaFever/golang.org-x-net,gomini/net,mckinney-subgraph/net,cesanta/goxnet,golang/net,raboof/net,cmars/tools,cmars/tools,yangxianjun/net,albertjin/golang-net,zabawaba99/net,cesanta/goxnet,DramaFever/golang.org-x-net,veliamsli/net,gomini/net,dafyddcrosby/net,muzining/net,subuk/net,cmars/tools,dafyddcrosby/net,w4ngyi/net,flowlo/go-net,flowlo/go-net,kiip/net,zabawaba99/net,mckinney-subgraph/net,subuk/net,veliamsli/net,cmars/tools