cmd/gomobile: accept -o for target=ios
cmd/gomobile: accept -o for target=ios

Fixes golang/go#11447.

Change-Id: I6298927eb7d434ec4865c3fa45dd9e4dede17d8c
Reviewed-on: https://go-review.googlesource.com/11968
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>

DanielWorld/mobile,freeman0211/mobile,steeve/mobile,zach-klippenstein/mobile,ppamorim/mobile,samanalysis/mobile,b54898533/mobile,ppamorim/mobile,samanalysis/mobile,golang/mobile,ichu501/mobile,zach-klippenstein/mobile,ppamorim/mobile,0x90sled/mobile,swisspol/mobile,goldcaddy77/mobile,tempbottle/mobile,xing/gomobile,golang/mobile,cainiaocome/mobile,ppamorim/mobile,freeman0211/mobile,zach-klippenstein/mobile,MaxCy330/mobile,zach-klippenstein/mobile,tempbottle/mobile,steeve/mobile,ppamorim/mobile,0x90sled/mobile,ichu501/mobile,xing/gomobile,cainiaocome/mobile,cainiaocome/mobile,freeman0211/mobile,qskycolor/mobile,swisspol/mobile,MaxCy330/mobile,0x90sled/mobile,swisspol/mobile,b54898533/mobile,freeman0211/mobile,xing/gomobile,goldcaddy77/mobile,0x90sled/mobile,ichu501/mobile,tempbottle/mobile,goldcaddy77/mobile,samanalysis/mobile,cainiaocome/mobile,steeve/mobile,0x90sled/mobile,swisspol/mobile,samanalysis/mobile,b54898533/mobile,DanielWorld/mobile,ichu501/mobile,qskycolor/mobile,DanielWorld/mobile,MaxCy330/mobile,swisspol/mobile,b54898533/mobile,lonphy23t/mobile,steeve/mobile,tempbottle/mobile,lonphy23t/mobile,xing/gomobile,lonphy23t/mobile,goldcaddy77/mobile,b54898533/mobile,DanielWorld/mobile,MaxCy330/mobile,lonphy23t/mobile,qskycolor/mobile,golang/mobile,qskycolor/mobile,tempbottle/mobile,MaxCy330/mobile,lonphy23t/mobile,cainiaocome/mobile,goldcaddy77/mobile,ichu501/mobile,xing/gomobile,samanalysis/mobile,golang/mobile